Polygon inside a circle

#include<stdio.h>
#include<math.h>
#define PI acos(-1)
main()
{
    double r,n,a,b,c;
    while(scanf("%lf%lf",&r,&n)!=EOF)
    {
        a=n*r*r;
        b=2*PI;
        b=sin(b/n);
        c=(a*b)/2;
        printf("%.3lf\n",c);
    }
    return 0;
}