[print(f.reduce(o.mul,f.reduce(lambda x,y:[x[0]-y[0],x[1]-y[1]],[[int(i)for i in input().split()],[int(i)for i in input().split()]])))for f,o in[[__import__('functools'),__import__('operator')]]]

[print((H-h)*(W-w))for H,W,h,w in[map(int,open(0).read().split())]]

[
    print(
        f.reduce(
            o.mul,
            f.reduce(
                lambda x, y: [x[0] - y[0], x[1] - y[1]],
                [[int(i) for i in input().split()], [int(i) for i in input().split()]],
            ),
        )
    )
    for f, o in [[__import__("functools"), __import__("operator")]]
]

[print((H - h) * (W - w)) for H, W, h, w in [map(int, open(0).read().split())]]

-[ -    print( -        f.reduce( -            o.mul, -            f.reduce( -                lambda x, y: [x[0] - y[0], x[1] - y[1]], -                [[int(i) for i in input().split()], [int(i) for i in input().split()]], -            ), -        ) -    ) -    for f, o in [[__import__("functools"), __import__("operator")]] -] +[print((H - h) * (W - w)) for H, W, h, w in [map(int, open(0).read().split())]]