def floor_sum(n, m, a, b):
    ans = 0
    while True:
        if a >= m or a < 0:
            ans += n * (n - 1) * (a // m) // 2
            a %= m
        if b >= m or b < 0:
            ans += n * (b // m)
            b %= m
        y_max = a * n + b
        if y_max < m: break
        n, b, m, a = y_max // m, y_max % m, a, m
    return ans  
a,b,c,d=map(int,input().split())
if a*d-b*c==0:
  print(-1)
  exit()
if a*d-b*c<0:
  a,b,c,d=c,d,a,b
l=0
r=10**18
while True:
  m=(l+r+1)//2
  if m*(a*d-b*c)<b*d:
    l=m
  else:
    r=m-1
  if l==r:
    break
ans0=floor_sum(l,2*b,2*a,b+2*a)
ans1=floor_sum(l,2*d,2*c,d+2*c)
print(l-(ans0-ans1))