#include <bits/stdc++.h>
using namespace std;
void initial() {}
int main() {
  initial();
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  long double r;
  cin >> r;
  long double circum = 2 * 3.14 * r;
  cout << fixed << setprecision(20) << circum << endl;
}
