#include<bits/stdc++.h>

#define rep(i, n) for(int i = 0; i < (n); ++i)

using namespace std;

using ll = long long;
using P = pair<int , int>;
  
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  int r; cin >> r;
  cout << r * 2 * 3.141592653589793 << endl;
}
