# 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:
    exit(print(-1))

if A * D > B * C:
    A, C = C, A
    B, D = D, B

N = B * D // (B * C - A * D)
r = floor_sum(N + 1, 2 * D, 2 * C, D) - floor_sum(N + 1, 2 * B, 2 * A, B) 
print(N - r)