#include <bits/stdc++.h>
using namespace std;
const long long MAX = 1000000000000000000;
const char END = '\n';
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  double R, Ans;
  cin >> R;
  Ans = 2 * 3.1415 * R;
  cout << Ans << END;
  return 0;
}
