uva 10432

#include<stdio.h>
#include<math.h>
int main()
{

    double pi=2*acos(0.00);
    double r,n,area,b;
    while(scanf("%lf%lf",&r,&n)==2)
    {
        b=sin((2*pi)/n);
    area=(r*r*n*b)/2;
    printf("%.3lf\n",area);
    }
    return 0;
}