#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  double p = 22.0 / (7.0);
  cout.precision(8);
  cout << fixed;
  cout << 2.0 * p * r * 1.0 << "\n";
  return 0;
}
