2697

def gcd(a, b)
  if a==0 then return [b, 0, 1]
  else
    (g, xa, xb) = gcd(b%a, a)
    return [g, xb - (b / a) * xa, xa]
  end
end
def diof_solve(a, b, c)
  if a == 0 and b == 0 then return [c == 0, 0, 0]
  elsif a == 0 then return [c % b == 0, 0, c / b, 0]
  elsif b == 0 then return [c % a == 0, c / a, 0, 0]
  else
    (g, xg, yg) = gcd(a, b)
    return [c % g == 0, xg * c / g, yg * c / g, g]
  end
end
(rx, ry, a, b) = gets().split().map { |i| i.to_i() }
(result, x0, y0, g) = diof_solve(a, b, ry - rx)
if a == 0 then puts max(-y0, -1)
elsif b == 0 then puts max(x0, -1)
else
  kl = -x0 * g / b
  kh = y0 * g / a
  kkl = -2000000000
  kkh = 2000000000
  if -x0 * g % b != 0 then kl += 1 end
  if y0 * g % a != 0 then kh += 1 end
  if b * g > 0 then kkl = [kkl, kl].max()
  else kkh = [kkh, kl].min() end
  if a * g > 0 then kkl = [kkl, kh].max()
  else kkh = [kkh, kh].min() end
  if kkl > kkh then puts -1
  elsif kkl == -2000000000
    puts x0 + kkh * b / g - y0 + kkh * a / g
  elsif kkl == 2000000000
    puts x0 + kkl * b / g - y0 + kkl * a / g
  else
    puts [x0 + kkh * b / g - y0 + kkh * a / g, x0 + kkl * b / g - y0 + kkl * a / g].min()
  end
end