#include <bits/stdc++.h>
using namespace std;
void speed() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
int main() {
  speed();
  int r;
  cin >> r;
  double ans = 6.28318530717958623200 * r;
  cout.precision(20);
  cout << fixed << ans;
  return 0;
}
