#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double tpi = 6.28318530717958623200;
  double x;
  cin >> x;
  double z = tpi * x;
  cout << setprecision(3) << fixed;
  cout << tpi << '\n';
  return 0;
}
