Input from keyboard and sub - Microprocessor

.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
    MOV AH,1
    INT 21H
    MOV BL,AL

    MOV AH,1
    INT 21H
    MOV CL,AL


    SUB BL,CL
    ADD BL,48

    MOV AH,2
    MOV DL,BL
    INT 21H

MAIN ENDP
END MAIN