# https://qiita.com/sounansya/items/51b39e0d7bf5cc194081
def floor_sum(n, m, a, b):
    a1, a2 = a // m, a % m
    s = n * (n - 1) // 2 * a1
    b1, b2 = b // m, b % m
    if a2 == 0:
        return s + b1 * n
    k = (a2 * (n - 1) + b2) // m
    return s + n * (k + b1) - floor_sum(k, a2, m, m + a2 - b2 - 1)

from math import gcd
a,b,c,d = map(int, input().split())
g = gcd(a, b)
a //= g
b //= g
g = gcd(c, d)
c //= g
d //= g
if (a, b) == (c, d):
    print(-1)
    exit()
if a * d > c * b:
    a,b,c,d = c,d,a,b
mx = b * d // (b * c - a * d)
k1 = floor_sum(mx+1, 2 * b, 2 * a, b)
k2 = floor_sum(mx+1, 2 * d, 2 * c, d)
print(mx - (k2 - k1))