#include <bits/stdc++.h>
using namespace std;
int MOD = 1000000007;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  double ret = (double)2.0 * 3.141592653 * n;
  cout << ret << endl;
  return 0;
}
