data segment 
	msg1 db 'Enter:','$'
    err  db 'enter error!','$'
	num1 dw ?
	num2 dw ?
	sum dw ?
	crlf 13,10,'$'

	flag db ?
	oper db ?
	
data ends
code segment
	main proc far
	assume cs:code,ds:data
start:
    push ds
    sub ax,ax
    push ax
    
    mov ax,data
    mov ds,ax
    
    mov ah,9
    lea dx msg1
    int 21h
    
    lea si,buffer
    mov flag,0
    mov oper,0
    mov bx,0
next:
    mov ah,01
    int 21h
    test flag,01h
    jnz follow
    
    sub al,30h   
    jl error   
    cmp al,9d   
    jg error 
    mov flag,1
    cbw   
    xchg ax,bx   
    mov cx,10d   
    mul cx   
    xchg ax,bx   
    add bx,ax
    jmp next:
follow:
     test oper,01h
     jnz follow2
     cmp al,2bh
     jnz next1
     mov oper,1
      mov num1,bx
      mov bx,0
     jmp next
 next1:
     cmp al,2dh
     jnz next2
     mov oper,2
     mov num1,bx
     mov bx,0
     jmp next
 next2:
     cmp al,2ah
     jnz next3
     mov oper,3
      mov num1,bx
      mov bx,0
     jmp next
 next3:
     cmp al,2fh
     jnz next4
     mov oper,4
      mov num1,bx
      mov bx,0
     jmp next
 next4:
     sub al,30h   
    jl error   
    cmp al,9d   
    jg error 
   cbw   
    xchg ax,bx   
    mov cx,10d   
    mul cx   
    xchg ax,bx   
    add bx,ax
   
follow2:
    cmp al,oah
    jz  exit
    sub al,30h   
    jl error   
    cmp al,9d   
    jg error 
    mov flag,1
    cbw   
    xchg ax,bx   
    mov cx,10d   
    mul cx   
    xchg ax,bx   
    add bx,ax
    jmp next
    
 error:
       mov ah,9
       lea dx,err
       int 21h
       jmp end1
    
 exit: mov num2,bx
 
 end1:
   ret
   main endp
 code ends
 end start