def egcd(a, b):
  if a == 0:
    return (0, 1)
  else:
    y, x = egcd(b % a, a)
    return (x - (b // a) * y, y)

A,B,a,b = map(int, input().split(' '))

x,y = egcd(A,B)
ans = a * B * y + b * A * x
print(ans)