#include<bits/stdc++.h>
using namespace std;
//#include <boost/multiprecision/cpp_int.hpp>
//using namespace boost::multiprecision;

#define int long long int

signed main(){

ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int t=1;
//cin>>t;
while(t--){
int r;
cin>>r;

cout<<2*(3.141)*r<<endl;


}


}

