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())
P, Q, R, S = 2 * A * D, 2 * B * C, B * D, 2 * B * D

if P == Q:
    print(-1)
    exit()
if P > Q: P, Q = Q, P
mx = (S + Q - P - 1) // (Q - P)
cntone = floor_sum(mx, S, Q, R) - floor_sum(mx, S, P, R)
print(mx - cntone - 1)