open Scanf
open Printf

let () =
  let s,t = scanf "%s %s" (fun s t -> (s, t)) in
  let s_length = String.length s in
  let rec loop s t i =
    if i > s_length -1 then 0
    else if s.[i] <> t.[i] then 1 + loop s t (i + 1)
                           else loop s t (i + 1)
  in
    printf "%d\n" (loop s t 0)