diff --git a/math_probability/sum_two/sum_two_solution.ipynb b/math_probability/sum_two/sum_two_solution.ipynb
index cfba2a3..62bfcaf 100644
--- a/math_probability/sum_two/sum_two_solution.ipynb
+++ b/math_probability/sum_two/sum_two_solution.ipynb
@@ -122,11 +122,14 @@
     "    def sum_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",
-    "        carry = (a&b) << 1\n",
-    "        if carry != 0:\n",
-    "            return self.sum_two(result, carry)\n",
-    "        return result;"
+    "        #max bit length, change to 32 for 32bit\n",
+    "        max_positive = 2 ** 64\n",
+    "        min_negative = -2 ** 64\n",
+    "        while b != 0:\n",
+    "		 if b == max_positive:\n",
+    "			return a ^ min_negative\n",
+    "		a,b = a ^ b,(a&b)<<1\n",
+    "    return a"
    ]
   },
   {