open Batteries

(* ===== MAIN ===== *)

let () =
  let s, t = Scanf.scanf " %s %s" (fun s t -> s, t) in
  let ans =
       List.combine (String.to_list s) (String.to_list t)
    |> List.filter (fun (a, b) -> a <> b)
    |> List.length
  in
    Printf.printf "%d\n" ans

