subtract 2 numbers in Assembly language

.model small
.stack 100h
.code
start:
mov ah,1h
int 21h
sub al,30h

mov bh,al

mov ah,1h
int 21h
sub al,30h

sub bh,al
add bh,30h
int 21h

mov dl,10
int 21h

mov dl,bh
mov ah,2h
int 21h

mov ah,4ch
int 21h
end start