#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int t, n, i, j, b, a, q, m;
  long double r;
  cin >> r;
  cout << 2.0 * 3.141 * r << "\n";
}
