#include <bits/stdc++.h>
using namespace std;
void solve() {
  double r;
  double pi = 3.14159265358979;
  cin >> r;
  cout << 2 * pi * r << '\n';
}
signed main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  solve();
  return 0;
}
