#include <bits/stdc++.h>
using namespace std;
void solve() {
  double n;
  cin >> n;
  cout << 2LL * 3.14 * n;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  solve();
  return 0;
}
