#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double r;
  double pi = 3.14159265358979323846264;
  cin >> r;
  r = 2 * pi * r;
  cout << setprecision(10) << r;
  return 0;
}
