#include <bits/stdc++.h>
using namespace std;
void lucifer() {
  long long int n;
  cin >> n;
  cout << 2 * (M_PI)*n;
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t = 1;
  cout << fixed << setprecision(14);
  while (t--) {
    lucifer();
  }
}
