(define (System.out.println x)
  (begin
    (display x)
    (newline)
  )
)

(let(
     (Len    (read))
     (Before (symbol->string (read)))
     (After  (symbol->string (read)))
     (CngCnt 0)
    )
  (begin
      (let loop((i 0))
        (begin
          (unless (string=? (substring Before i (+ i 1)) (substring After i (+ i 1)))
            (set! CngCnt (+ CngCnt 1))
          )
          (unless (= i (- Len 1))
            (loop (+ i 1))
          )
        )
     )
    (System.out.println CngCnt)
  )
)