#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int R;
  cin >> R;
  cout << fixed << setprecision(10) << 2 * R * 3.1415926535 << endl;
  return 0;
}
