#include <bits/stdc++.h>
using namespace std;
double R;
const double PI = 3.141592653589793;
void solve() {
  cin >> R;
  cout << setprecision(18) << fixed << 2 * PI * R << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int T = 1;
  for (int t = 1; t <= T; t++) {
    solve();
  }
  return 0;
}
