INPUT "Input Charge Card Number"; A$
FOR X = 1 TO LEN(A$)
IF MID$(A$, X, 1) >= "0" AND MID$(A$, X, 1) <= "9" THEN B$ = B$ + MID$(A$, X, 1)
NEXT X
FOR X = LEN(B$) - 1 TO 1 STEP -1
IF M = 2 THEN M = 1 ELSE M = 2
IF VAL(MID$(B$, X, 1)) * M > 9 THEN CS = CS + VAL(MID$(B$, X, 1)) * M - 9 ELSE CS = CS + VAL(MID$(B$, X, 1)) * M
NEXT X
IF (CS + VAL(RIGHT$(B$, 1))) MOD 10 = 0 THEN PRINT "OK": END ELSE PRINT "Error"