#include <bits/stdc++.h>
using namespace std;
void solve();
int main(int argc, char const *argv[]) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  solve();
}
void solve() {
  double R;
  cin >> R;
  double pie = 6.28318530717958623200 / 2;
  printf("%.8f\n", 2 * pie * R);
}
