import sys, itertools
 
sys.setrecursionlimit(500005)
stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()

a, b, n, m = na()

def con(a, b, n):
    if (a - b) % n == 0:
        return (a - b) // n -1
    
    else:
        return (a - b) // n 

if a < b:
    c = con(b, a, m)
    a += c + 1
    b -= m * (c + 1)
    ans = a - 1

    c = con(a, b, n)
    if c != 0:
        d = a - (n * (c + 1))
        a = b + c
    else:
        d = 0
        a = 0
    
        
elif a > b:
    c = con(a, b, n)
    a -= n * (c + 1)
    b += c + 1
    ans = b - 1

    c = con(b, a, m)
    if c != 0:
        b = a + c
        d = b - (m * (c + 1))
    else:
        b = 0
        d = 0

print(max(a, b, ans, d))