#include <bits/stdc++.h>
const double PI = 3.141592653589793238463;
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    float n;
    cin >> n;
    cout << 2 * M_PI * n << endl;
    ;
  }
  return 0;
}
