#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n;
  cin >> n;
  double pie = 6.28318530717958623200;
  pie /= 2;
  pie *= 2;
  cout << setprecision(10) << fixed << pie << endl;
}
int main() {
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
