#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  if (fopen("input.txt", "r"))
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
  ios::sync_with_stdio(0), cin.tie(0);
  long double r;
  cin >> r;
  long double pi = M_PI;
  cout << setprecision(20) << 2 * pi * r;
}
