#include<stdio.h>
#include<math.h>
int main()
{
	double c, rin, rout;
	scanf("%lf%lf%lf", &c, &rin, &rout);
	double R = (rin + rout) / 2, r = (rout - rin) / 2;
	printf("%.20lf", 2 * acos(-1) * acos(-1) * R * r * r * c);
	return 0;
}