let rec pow a = function
  | 0 -> 1
  | 1 -> a
  | n ->
    let b = pow a (n / 2) in
    b * b * (if n mod 2 = 0 then 1 else a)

let rec powmod a d = function
  | 0 -> 1
  | 1 -> a mod d
  | n ->
    let b = (powmod a d (n / 2)) mod d in
    (((b * b) mod d) * (if n mod 2 = 0 then 1 else a)) mod d