#include <bits/stdc++.h>
using namespace std;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  long double PI = 3.141592653589793238;
  int r;
  cin >> r;
  cout << setprecision(10) << fixed;
  cout << 2 * r * PI << "\n";
  return 0;
}
