N = int(input())
M = int(input())
x = int(input())
y = int(input())
if N > M:
    a = M
    b = N
else:
    a = N
    b = M
x1 = a - x
y1 = b - y
if x <= y:
    if x <= x1:
        if x <= y1:
            print(x)
        else:
            print(y1)
    else:
        if x1 <= y1:
            print(x1)
        else:
            print(y1)
else:
    if y <= x1:
        if y <= y1:
            print(y)
        else:
            print(y1)
    else:
        if x1 <= y1:
            print(x1)
        else:
            print(y1)