#include <bits/stdc++.h>
using namespace std;
void solve() {
  double r;
  cin >> r;
  cout << fixed << setprecision(16) << 2 * 3.1415926535 * r << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  solve();
  return 0;
}
