#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
using namespace std;
int main(){
    double RIN,ROUT,C,R,r,men,seki;
    cin >> C;
    cin >> RIN >> ROUT;
    R = (RIN+ROUT)/2;
    r = (ROUT-RIN)/2;
    men = r*r*M_PI;
    seki = men*2*R*M_PI;
    printf("%0.7f",C*seki);
    return 0;
}