#include <bits/stdc++.h>
const long long mod = (long long)1e9 + 7;
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t;
  cin >> t;
  long long pi = 3.1415;
  long long a = 2 * pi * t;
  cout << floor(a) << "\n";
  return 0;
}
