a,b,n,m=map(int,input().split())
L=-1;R=10**18+1
while R-L>1:
  x=(L+R)//2
  f=0
  if x<=a and x<=b:
    f=1
  elif x<=a and x>b:
    if b+(a-x)//n>=x:
      f=1
  elif x>a and x<=b:
    if a+(b-x)//m>=x:
      f=1
  if f==0:
    R=x
  else:
    L=x
print(L)