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

struct Precision{
  Precision(){
    cout<<fixed<<setprecision(12);
  }
}precision_beet;

//INSERT ABOVE HERE
signed main(){
  double c,r1,r2;
  cin>>c>>r1>>r2;
  double R=(r1+r2)/2,r=(r2-r1)/2;
  double PI=acos(-1);
  cout<<c*(PI*r*r)*(PI*2*R)<<endl;
  return 0;
}