// yukicoder: No.89 どんどんドーナツどーんといこう
// 2019.4.12 bal4u

#include <stdio.h>

#define PI 3.1415926535897932384626433832795
int C, Rin, Rout;

int main()
{
	scanf("%d%d%d", &C, &Rin, &Rout);
	printf("%.12lf\n", 
		C*PI*PI / 4 * (Rin+Rout)*(Rout-Rin)*(Rout-Rin));
	return 0;
}