Fibonacci

ll Fib(int n) {
  if (n == 0 || n == 1) return 1ll;
  ll prev = 1ll;
  ll current = 1ll;
  for (int i = 0; i < n - 1; ++i) {
    ll next = prev + current;
    prev = current;
    current = next;
  }
  return current;
}

int main() {
  int n = 0;
  std::cin >> n;
  std::cout << Fib(n);
}