#include <bits/stdc++.h>
using namespace std;
void solve() {
  int r;
  cin >> r;
  cout << fixed << setprecision(3) << 2 * 3.141592 * r;
}
int main() {
  int t = 1;
  for (int i = 0; i < t; i++) {
    solve();
  }
  return 0;
}
