There are H rows and W columns of white square cells.

You will choose h of the rows and w of the columns, and paint all of the cells contained in those rows or columns.

How many white cells will remain?

It can be proved that this count does not depend on what rows and columns are chosen.

Constraints

* All values in input are integers.
* 1 \leq H, W \leq 20
* 1 \leq h \leq H
* 1 \leq w \leq W

Input

Input is given from Standard Input in the following format:


H W
h w


Output

Print the number of white cells that will remain.

Examples

Input

3 2
2 1


Output

1


Input

5 5
2 3


Output

6


Input

2 4
2 4


Output

0
#!/usr/bin/env python2
"""
This file is part of https://github.com/cheran-senthil/PyRival
Copyright 2019 Cheran Senthilkumar <hello@cheran.io>

"""
from __future__ import division, print_function

import itertools
import os
import sys
from atexit import register
from io import BytesIO


class dict(dict):
    """ dict() -> new empty dictionary """

    def items(self):
        """ D.items() -> a set-like object providing a view on D's items """
        return dict.iteritems(self)

    def keys(self):
        """ D.keys() -> a set-like object providing a view on D's keys """
        return dict.iterkeys(self)

    def values(self):
        """ D.values() -> an object providing a view on D's values """
        return dict.itervalues(self)


def gcd(x, y):
    """ greatest common divisor of x and y """
    while y:
        x, y = y, x % y
    return x


range = xrange

filter = itertools.ifilter
map = itertools.imap
zip = itertools.izip

sys.stdin = BytesIO(os.read(0, os.fstat(0).st_size))
sys.stdout = BytesIO()
register(lambda: os.write(1, sys.stdout.getvalue()))

input = lambda: sys.stdin.readline().rstrip('\r\n')


def main():
    H, W = map(int, input().split())
    h, w = map(int, input().split())

    print(H * W - (h * W + H * w - h * w))


if __name__ == '__main__':
    main()
