#include <bits/stdc++.h>
using namespace std;
const long double PIE = atan2(0, -1);
void solve() {
  long double r;
  cin >> r;
  long double ans = 2.00 * PIE * r;
  cout.precision(100);
  cout << ans << "\n";
}
int main() {
  solve();
  return 0;
}
