import math


def CRT(rem_pair, mod_pair):
    """中国剰余定理
    - rem_pair = (r1, r2)
    - mod_pair = (m1, m2)
    - m1 != 0 and m2 != 0

    x = m1 (mod r1)
    x = m2 (mod r2)
    となる整数xが存在する場合0以上の最小の自然数を返す(存在しない場合は-1)
    """
    r1, r2 = rem_pair
    m1, m2 = mod_pair

    assert (m1 != 0 and m2 != 0)
    assert len(rem_pair) == len(mod_pair) == 2
    g = math.gcd(m1, m2)
    if (r2 - r1) % g != 0:
        return -1
    M1, M2, R = m1//g, (-m2)//g, (r2 - r1)//g

    inv = pow(M1, -1, M2) * R % M2

    return (m1*inv + r1) % abs(m1*m2//g)


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