#define _USE_MATH_DEFINES
#include <iomanip>
#include <iostream>
#include <cmath>
using namespace std;
int main() {
	double C;
	double Rin, Rout;
	double r1, r2;
	cin >> C >> Rin >> Rout;
	r1 = (Rout - Rin) / 2.0;
	r2 = Rin + r1;
	cout << fixed << setw(10)<< C *((M_PI * r1 *r1) * (2 * M_PI * r2)) << endl;
	return 0;
}