#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 7;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  double r;
  cin >> r;
  cout << fixed << setprecision(20) << 2.0 * acos(-1) * r;
  return 0;
}
