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

int main(){
    double c,rin,rout; cin >> c >> rin >> rout;
    double r = (rout - rin ) / 2;
    double d = (rin + rout) / 2;
    double ans = r*r*M_PI*2*M_PI*d*c;
    printf("%f\n",ans);
}