#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  double r, i, j, n, x;
  cin >> r;
  r = (22.00 * r) / 7.00;
  std::cout << std::fixed;
  cout << std::setprecision(2) << r << '\n';
}
