Нахождение максимальной цифры и построение минимального числа — QBasic(Бейсик)

CLS
maxchislo = 2147483647
DIM b$(255)
DO
    PRINT "Nakhozhdenie maksimalnoi tsifri i postroenie minimalnogo chisla"
    PRINT "iz tsifr iskhodnogo chisla"
    10 INPUT "Vvedite naturalnoe chislo:", a$
    f = 0
    k = 0
    FOR i = 1 TO LEN(a$)
        k = k + 1
        b$(k) = MID$(a$, i, 1)
        IF b$(k) < "0" OR b$ > "9" THEN f = 1
    NEXT
    IF f = 0 AND VAL(a$) > maxchislo THEN f = 1
    IF f = 1 THEN PRINT "Vi vveli otritsatelnoe chislo ili desyatichnuyu drob."
    IF f = 1 THEN PRINT "Povtorite vvod."
    IF f = 1 THEN GOTO 10
LOOP UNTIL f = 0
max$ = b$(1)
kol = 1
FOR i = 2 TO k
    IF b$(i) = max$ THEN kol = kol + 1
    IF b$(i) > max$ THEN max$ = b$(i): kol = 1
NEXT
PRINT "Maksimalnaya tsifra="; max$
PRINT "Ona vstrechalas"; kol; "raz"
FOR i = 1 TO k
    FOR j = 2 TO k
        IF b$(j) < b$(j - 1) THEN SWAP b$(j), b$(j - 1)
    NEXT
NEXT
FOR i = 1 TO k
    c$ = c$ + b$(i)
NEXT
PRINT "Minimalnoe chislo:"; VAL(c$)