#include <bits/stdc++.h>   const double PI = acos(-1.0);   int main() {     double n, r;     scanf( %lf%lf , &n, &r);     double p1 = PI / n, p2 = PI / (2 * n), p3 = PI - p1 - p2;     double l3 = r, l2 = l3 / sin(p3) * sin(p2);     double s = l3 * l2 * sin(p1);     printf( %.15f n , n * s);     return 0;   }   