#include <stdio.h>

int main(void)
{
	double c, i, o, t, p = 3.141592653589793;
	scanf("%lf%lf%lf", &c, &i, &o);
	t = .5 * (o - i);
	printf("%.6f", t * t * (i + t) * p * p * c * 2);
	return 0;
}