A,B,a,b = map(int,input().split())


if (a == A and b == B) or (a==0 and b==0):
    print(0)

else :
    

    for x in range(1, 2**10000):
    	
    	aa = x%A
    	bb = x%B
    	if aa == a and bb  == b:
    		print(x)
    		break