#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ;
  double n;
  cin >> n;
  cout << setprecision(10) << n * 6.28318530717958623200;
  return 0;
}
