#include <bits/stdc++.h>
using namespace std;
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  double n;
  cin >> n;
  double pi = 6.28318530717958623200;
  double ans = pi * n;
  cout << fixed << setprecision(20) << ans;
}
