diff --git a/math_probability/sub_two/__init__.py b/math_probability/sub_two/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/math_probability/sub_two/sub_two_challenge.ipynb b/math_probability/sub_two/sub_two_challenge.ipynb
new file mode 100644
index 0000000..704e278
--- /dev/null
+++ b/math_probability/sub_two/sub_two_challenge.ipynb
@@ -0,0 +1,167 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Challenge Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Find the difference of two integers without using the + or - sign.\n",
+    "\n",
+    "* [Constraints](#Constraints)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)\n",
+    "* [Solution Notebook](#Solution-Notebook)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Constraints\n",
+    "\n",
+    "* Can we assume the inputs are valid?\n",
+    "    * No, check for None\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "<pre>\n",
+    "* None input -> TypeError\n",
+    "* 7, 5 -> 2\n",
+    "* -5, -7 -> 2\n",
+    "* -5, 7 -> -12\n",
+    "* 5, -7 -> 12\n",
+    "</pre>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "Refer to the [Solution Notebook]().  If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class Solution(object):\n",
+    "\n",
+    "    def sub_two(self, val):\n",
+    "        # TODO: Implement me\n",
+    "        pass"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**The following unit test is expected to fail until you solve the challenge.**"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "# %load test_sub_two.py\n",
+    "from nose.tools import assert_equal, assert_raises\n",
+    "\n",
+    "\n",
+    "class TestSubTwo(object):\n",
+    "\n",
+    "    def test_sub_two(self):\n",
+    "        solution = Solution()\n",
+    "        assert_raises(TypeError, solution.sub_two, None)\n",
+    "        assert_equal(solution.sub_two(7, 5), 2)\n",
+    "        assert_equal(solution.sub_two(-5, -7), 2)\n",
+    "        assert_equal(solution.sub_two(-5, 7), -12)\n",
+    "        assert_equal(solution.sub_two(5, -7), 12)\n",
+    "        print('Success: test_sub_two')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestSubTwo()\n",
+    "    test.test_sub_two()\n",
+    "\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    main()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Solution Notebook\n",
+    "\n",
+    "Review the [Solution Notebook]() for a discussion on algorithms and code solutions."
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.5.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/math_probability/sub_two/sub_two_solution.ipynb b/math_probability/sub_two/sub_two_solution.ipynb
new file mode 100644
index 0000000..4593615
--- /dev/null
+++ b/math_probability/sub_two/sub_two_solution.ipynb
@@ -0,0 +1,206 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Solution Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Find the difference of two integers without using the + or - sign.\n",
+    "\n",
+    "* [Constraints](#Constraints)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Constraints\n",
+    "\n",
+    "* Can we assume the inputs are valid?\n",
+    "    * No, check for None\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "<pre>\n",
+    "* None input -> TypeError\n",
+    "* 7, 5 -> 2\n",
+    "* -5, -7 -> 2\n",
+    "* -5, 7 -> -12\n",
+    "* 5, -7 -> 12\n",
+    "</pre>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll look at the following example, subtracting a and b:\n",
+    "\n",
+    "<pre>\n",
+    "a 0110 = 6 \n",
+    "b 0101 = 5\n",
+    "</pre>\n",
+    "\n",
+    "First, subtract a and b, without worrying about the borrow (0-0=0, 0-1=1, 1-1=0):\n",
+    "\n",
+    "result = a ^ b = 0011\n",
+    "\n",
+    "Next, calculate the borrow (0-1=1).  We'll need to left shift one to prepare for the next iteration when we move to the next most significant bit:\n",
+    "\n",
+    "~a     = 1001\n",
+    " b     = 0101\n",
+    "~a & b = 0001\n",
+    "\n",
+    "borrow = (~a&b) << 1 = 0010\n",
+    "\n",
+    "If the borrow is not zero, we'll need to subtract the borrow from the result.  Recusively call the function, passing in result and borrow.\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(b), where b is the number of bits\n",
+    "* Space: O(b), where b is the number of bits"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class Solution(object):\n",
+    "\n",
+    "    def sub_two(self, a, b):\n",
+    "        if a is None or b is None:\n",
+    "            raise TypeError('a or b cannot be None')\n",
+    "        result = a ^ b;\n",
+    "        borrow = (~a & b) << 1\n",
+    "        if borrow != 0:\n",
+    "            return self.sub_two(result, borrow)\n",
+    "        return result;"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Overwriting test_sub_two.py\n"
+     ]
+    }
+   ],
+   "source": [
+    "%%writefile test_sub_two.py\n",
+    "from nose.tools import assert_equal, assert_raises\n",
+    "\n",
+    "\n",
+    "class TestSubTwo(object):\n",
+    "\n",
+    "    def test_sub_two(self):\n",
+    "        solution = Solution()\n",
+    "        assert_raises(TypeError, solution.sub_two, None)\n",
+    "        assert_equal(solution.sub_two(7, 5), 2)\n",
+    "        assert_equal(solution.sub_two(-5, -7), 2)\n",
+    "        assert_equal(solution.sub_two(-5, 7), -12)\n",
+    "        assert_equal(solution.sub_two(5, -7), 12)\n",
+    "        print('Success: test_sub_two')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestSubTwo()\n",
+    "    test.test_sub_two()\n",
+    "\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    main()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": false,
+    "scrolled": true
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Success: test_sub_two\n"
+     ]
+    }
+   ],
+   "source": [
+    "%run -i test_sub_two.py"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.5.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/math_probability/sub_two/test_sub_two.py b/math_probability/sub_two/test_sub_two.py
new file mode 100644
index 0000000..f022a3f
--- /dev/null
+++ b/math_probability/sub_two/test_sub_two.py
@@ -0,0 +1,22 @@
+from nose.tools import assert_equal, assert_raises
+
+
+class TestSubTwo(object):
+
+    def test_sub_two(self):
+        solution = Solution()
+        assert_raises(TypeError, solution.sub_two, None)
+        assert_equal(solution.sub_two(7, 5), 2)
+        assert_equal(solution.sub_two(-5, -7), 2)
+        assert_equal(solution.sub_two(-5, 7), -12)
+        assert_equal(solution.sub_two(5, -7), 12)
+        print('Success: test_sub_two')
+
+
+def main():
+    test = TestSubTwo()
+    test.test_sub_two()
+
+
+if __name__ == '__main__':
+    main()
\ No newline at end of file