import math
A,B,N,M = map(int, input().split())
if A < B:
    num = math.ceil(B/M)
    A += num
    B = B - num*M
    while 1:
        if A > B:
            A -= 1
            B = B + M
        else:
            break
    print(A)
elif A > B:
    num = math.ceil(A/N)
    B += num
    A = A - N*num
    while 1:
        if A < B:
            B -= 1
            A = A + N
        else: break
    print(B)