algorithm to simulate multiplication by addition

Question

How to design an algorithm to simulate multiplication by addition. input two integers. they may be zero, positive or negative..

Answer 1

def multiply(a, b):                                                                                                                                                               
    if (a == 1):
        return b
    elif (a == 0):
        return 0
    elif (a < 0):
        return -multiply(-a, b)
    else:
        return b + multiply(a - 1, b)