#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const double PI = 3.14159265359;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  double r;
  cin >> r;
  cout << 2 * PI * r;
  return 0;
}
