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

int main() {
    double C, R, r;
    cin >> C >> r >> R;

    const double pi = 3.141592653589793;

    cout << fixed << setprecision(10) << C * (pow((R - r) / 2.0, 2.0) * pi * (R + r) * pi) << endl;
}