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

int main(){
    int C, r1, r2;
    double S, V, C_sum;

    cin >> C >> r1 >> r2;

    S = M_PI*((r2 - r1)/2.0)*((r2 - r1)/2.0);
    V = 2*M_PI*(r1 + (r2 - r1)/2.0)*S;
    C_sum = C*V;

    printf("%.5f\n", C_sum);

    return 0;
}