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18 lines
387 B
C++
18 lines
387 B
C++
#include <cstdio>
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#include <cmath>
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const int MAXN = 200000 + 5;
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double t[MAXN];
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int main()
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{
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t[0] = acos(-1) / 2., t[1] = 1.;
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for (int i = 2; i < MAXN; ++ i) {
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t[i] = t[i - 2] * (i - 1) / i;
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}
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int n, R;
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while (scanf("%d%d", &n, &R) == 2) {
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double res = .5 * t[n + 1] * R / t[2];
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printf("%.10lf\n", res);
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}
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return 0;
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}
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