def floor_sum(n, m, a, b):
    ans = 0
    while True:
        if a < 0 or a >= m:
            k = a // m
            a = a % m
            if a < 0:
                a += m
                k -= 1
            ans += k * n * (n - 1) // 2
        if b < 0 or b >= m:
            k = b // m
            b = b % m
            if b < 0:
                b += m
                k -= 1
            ans += k * n
        y_max = (a * n + b) // m
        if y_max == 0:
            break
        x_max = y_max * m - b
        ans += (n - (x_max + a - 1) // a) * y_max
        n, m, a, b = y_max, a, m, (a - x_max % a) % a
    return ans

A, B, C, D = map(int, input().split())
if A * D == B * C:
    print(-1)
    exit()
if A * D > B * C:
    A, C = C, A
    B, D = D, B
Ntop = (D * B + C * B - A * D - 1) // (C * B - A * D)

fla = floor_sum(Ntop, 2 * B, 2 * A, B)
flb = floor_sum(Ntop, 2 * D, 2 * C, D)
print(Ntop - (flb - fla) - 1)