#include<bits/stdc++.h>
using namespace std;
//<<setprecision(30)
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rep1(i, n) for (ll i = 1; i < (ll)(n); i++)
#define INF 1000000000000
#define MOD 1000000007
using ll = long long;
using Graph = vector<vector<int>>;

int main(){
  double R; cin>>R;
  doubel pi = 6.28318530717958623200;
  cout<<R*pi<<endl;
}