.MODEL SMALL
.STACK
.DATA
    msg   db 'input number?$'
    msg1  db '1+2+...+$'
    msg2  db 'error input!!! retry it! $'
    odoa  db 0dh,0ah,'$'
    t1    db 20 dup (?)
    tt    db 20 dup (?)
    tab   db '0123456789'
.CODE
    MOV   AX,@DATA
    MOV   DS,AX
;
tryagain:
    lea   dx,msg
    mov   ah,09
    int   21h
    xor   si,si
;
rea:
    mov   ah,01    ; input from kb
    int   21h
    cmp   al,13    ; check enter!!!
    je    proc1
    cmp   al,'0'   ; check number!!!
    jl    err
    cmp   al,'9'
    jg    err
;
    mov   tt[si],al
    inc   si
    jmp   rea
err:
    lea   dx,odoa
    mov   ah,09
    int   21h
;
    lea   dx,msg2
    mov   ah,09h
    int   21h
    jmp   tryagain
;
    lea   dx,odoa
    mov   ah,09
    int   21h
;
proc1:
    lea   dx,odoa
    mov   ah,09
    int   21h
;
    lea   dx,msg1
    mov   ah,09
    int   21h
;
    mov   tt[si],'$'    ; for ending
    lea   dx,tt
    mov   ah,09
    int   21h
;
    mov   dl,'='
    mov   ah,02     ; print '='
    int   21h
;
    lea   dx,odoa
    mov   ah,09
    int   21h
;
    mov   ah,4ch
    int   21h
END