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 == c * b:
  print(-1)
  exit()

if a * d > c * b:
  a, b, c, d = c, d, a, b

n = b * d // (c * b - a * d)
ans = n - 1
ans -= floor_sum(n, 2*d, 2*c, d) - floor_sum(n, 2*b, 2*a, b)
print(ans)