#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
 
int main() {
     ios::sync_with_stdio(false);
     ll t;
     cin>>t;
     ll pi=3.1414;
    ll area;
  area=2*pi*t;
  cout<<area<<endl;
     }
	return 0;
}
 
