#include "bits/stdc++.h"
using namespace std;

int main() {
	double C, Rin, Rout, Rc, Rmid;
	const double PI = 3.14159265358979;
	cin >> C >> Rin >> Rout;
	Rc = (Rout - Rin) / 2;
	Rmid = (Rin + Rout) / 2;
	printf("%.6lf\n", C * (Rc * Rc * PI) * (Rmid * 2 * PI));
}