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) exit(0) if a * d < b * c: a, b, c, d = c, d, a, b ans = 0 m = b * d // (a * d - b * c) ng, ok = 0, 10 ** 18 + 1 while ok - ng > 1: md = (ok + ng) // 2 if ((2 * md + 1) * b - 1) // (2 * a) + 1 >= m: ok = md else: ng = md n2 = ok ng, ok = 0, 10 ** 18 + 1 while ok - ng > 1: md = (ok + ng) // 2 if ((2 * md + 1) * d - 1) // (2 * c) + 1 >= m: ok = md else: ng = md n1 = ok l = floor_sum(n2, 2 * a, 2 * b, b - 1) + n2 r = floor_sum(n1, 2 * c, 2 * d, d - 1) + n1 + m * (n2 - n1) print(m - (r - l) - 1) # cr, cl = 0, 0 # n = 0 # while 1: # r = ((2 * n + 1) * d - 1) // (2 * c) + 1 # cr += min(r, m) # l = ((2 * n + 1) * b - 1) // (2 * a) + 1 # cl += min(l, m) # t = min(r, m) - min(l, m) # if l >= m: # break # ans += t # n += 1 # print(cl, cr)