module Hamming

let hamming s1 s2 =
    let pairs = Seq.zip s1 s2
    Seq.fold (fun a x -> if (x |> fst) <> (x |> snd) then (a + 1) else a) 0 pairs 

let distance (strand1: string) (strand2: string): int option =
    if String.length strand1 <> String.length strand2 then None else Some(hamming strand1 strand2)