#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  int r;
  cin >> r;
  cout << fixed << setprecision(15) << 2 * r * 3.141592 << endl;
  return 0;
}
