a, b, n, m = map(int, input().split())

if a == b:
    ans = a
elif a > b:
    x = (a-b) // n
    b += x
    a -= x * n
    while a < b:
        a += n
        b -= 1
    ans = b
else:
    x = (b-a) // m
    a += x
    b -= x * m
    while b < a:
        b += m
        a -= 1
    ans = a

print(ans)