How to add read and add hexadecimal numbers using machine language in pep/9

Question

BR main
sum: .WORD 0
num1: .BLOCK 1
num2: .BLOCK 1
num3: .BLOCK 1

main: LDWA sum,d 
DECI num1,d
ADDA num1,d

DECI num2,d
ADDA num2,d

DECI num3,d
ADDA num3,d

STWA sum,d 
DECO sum,d
STOP
.END

This is a machine language program written to add 3 integers in pep/9 and it is working fine.

I want to add a negative number ex: -3 to the two numbers without using subtraction.

But the conditions are :

• Store the -3 in hexadecimal.
• Do not use the subtract,negate,or invert instruction

EXPECTED INPUT : 2, 4, -3;
EXPECTED OUTPUT: 3

How can I do this?