#include <iostream>

const double pi = 3.14159265358979323846;

int main(){
	double c, r1, r2;
	std::cin >> c >> r1 >> r2;
	double a = (r2 - r1) / 2, b = (r1 + r2) / 2;
	printf("%.10lf\n", c * a * a * b * 2 * pi * pi);
	return 0;
}