#include <bits/stdc++.h>
using namespace std;
int main() {
	double C;
	double R_in, R_out, r, R;
	cin >> C >> R_in >> R_out;
	r = (R_out - R_in) / 2; R = (R_in + R_out) / 2;
	cout << fixed << setprecision(7) << 
		(C * M_PI * r * r * 2 * M_PI * R) << endl;
	return 0;
}