def inv_gcd(a, b):
    a = a % b
    if a == 0:
        return (b, 0)
    s = b
    t = a
    m0 = 0
    m1 = 1
    while t:
        u = s // t
        s -= t * u
        m0 -= m1 * u
        s, t = t, s
        m0, m1 = m1, m0
    if m0 < 0:
        m0 += b // s
    return (s, m0)


def inv_mod(x, m):
    assert 1 <= m
    z = inv_gcd(x, m)
    assert z[0] == 1
    return z[1]


def crt(r, m):
    assert len(r) == len(m)
    n = len(r)
    r0 = 0
    m0 = 1
    for i in range(n):
        assert 1 <= m[i]
        r1 = r[i] % m[i]
        m1 = m[i]
        if m0 < m1:
            r0, r1 = r1, r0
            m0, m1 = m1, m0
        if m0 % m1 == 0:
            if r0 % m1 != r1:
                return (0, 0)
            continue
        g, im = inv_gcd(m0, m1)
        u1 = m1 // g
        if (r1 - r0) % g:
            return (0, 0)
        x = (r1 - r0) // g % u1 * im % u1
        r0 += x * m0
        m0 *= u1
        if r0 < 0:
            r0 += m0
    return (r0, m0)


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())
A*=2
B*=2
C*=2
D*=2
if A*D==B*C:
    print(-1)
    exit()
A,B,C,D=A*D,B*D,C*B,D*B
mx=B//abs(A-C)
now=floor_sum(mx,B,A,A+B//2)-floor_sum(mx,D,C,C+D//2)
print(mx-abs(now))