#include<bits/stdc++.h>

using namespace std;

typedef long long int ll;

const ll mod = 1e9 + 7;
const ll MAXN = 1e5 + 5;

int main(){
  ios_base::sync_with_stdio(false);
  cin.tie(0);cout.tie(0);

  double r;
  cin >> r;
  double ans = 2.0 *  3.14 * r;
  cout << ans << endl;
}
