# 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)

a, b, c, d = map(int, input().split())
if a * d == b * c:
    print(-1)
else:
    if a * d > b * c:
        na, nb = c, d
        nc, nd = a, b
        a, b = na, nb
        c, d = nc, nd
    
    x0 = d * b // (b * c - a * d)
    s1 = floor_sum(x0 + 1, 2 * d, 2 * c, d)
    s2 = floor_sum(x0 + 1, 2 * b, 2 * a, b)
    s = s1 - s2
    
    print(x0 - s)