let diff s t =
  let n = String.length s in
  let rec iter i res =
    if i = n
    then res
    else
      if s.[i] = t.[i]
      then iter (i+1) res
      else iter (i+1) (res+1)
  in iter 0 0

let () = Scanf.scanf "%s %s" @@ fun s t -> Printf.printf "%d\n" @@ diff s t