#include <iostream>
#include <cmath>
using namespace std;

const double pi = acos(-1);

int main(){
    int C, Rin, Rout;
    cin >> C >> Rin >> Rout;
    
    double r = (Rout - Rin) / 2.0;
    double o = (Rout + Rin) / 2.0;

    double V = 2 * pi * pi * r * r * o;
    double res = V * C;
    
    printf("%.10f\n", res);
    
    return 0;
}