let rec fib n =
  match n with
  | 1 | 2 -> 1
  | _ -> (fib (n-1)) + (fib (n-2))
let fast_fib n =
  let rec h n pp p =
    match n with
    | 1 -> p
    | _ -> h (n-1) p (pp+p)
  in h n 0 1
let rec first_neg_fib n =
  let acc = fast_fib n in
  if acc < 0 then n
  else first_neg_fib (n+1)