#include <cstdio>
#define pi 3.14159265258979
int main(){
	float c, in, out, r;
	scanf("%f %f %f", &c, &in, &out);
	r = (out - in) / 2;
	c = 2 * pi * pi * r * r * (out-r) * c;
	printf("%.9f\n", c);
}