Adding and substracting in misp

ex7.asm

         .text 	  # code section
main:   
	li   	$v0, 4
     	la   	$a0, a
     	syscall
     	li   	$v0, 5
     	syscall 
	move 	$t0, $v0      #load value a to $t0
	li   	$v0, 4
     	la   	$a0, b
     	syscall
     	li   	$v0, 5
     	syscall 
	move 	$t1, $v0      #load value a to $t1
	li   	$v0, 4
     	la   	$a0, c
     	syscall
     	li   	$v0, 5
     	syscall 
	move 	$t2, $v0      #load value a to $t2
	li   	$v0, 4
     	la   	$a0, d
     	syscall
     	li   	$v0, 5
     	syscall 
	move 	$t3, $v0      #load value a to $t3
	add 	$t4, $t0, $t1 # $t4 = a + b
	sub 	$t5, $t2, $t3 # $t5 = c-d
	sub 	$t6, $t4, $t5 # $t6 = $t4-$t5
	li 	$v0, 1 	      #syscall to print int
	move 	$a0, $t6      #value to print
	syscall
	li 	$v0, 10       #exit
	syscall

        .data
a:	.asciiz "Give a:"
b:	.asciiz "Give b:"
c:	.asciiz "Give c:"
d:	.asciiz "Give d:"