AtCoder Beginner Contest 121 [ A - White Cells ] https://atcoder.jp/contests/abc121/tasks/abc121_a

abc121_a.py

'''

'''

import sys

sys.setrecursionlimit(10 ** 6)  # 再帰上限の引き上げ
input = sys.stdin.readline

INF = 2 ** 63 - 1

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

white = H*W
black = h*W
ans = (white-black) - (H-h)*w
print(ans)

'''
# 読み込み系テンプレ
N, M = map(int, input().split())
N = int(input())
h_list = list(map(int, input().split()))

sa = list(input())  # 一文字ずつバラす

N = int(input())
A = [tuple(map(int, input().split())) for i in range(N)]
# タプルの2番目の要素でソート
A.sort(key=lambda x: x[1], reverse=True)
# 昔のINF
INF = 10**18+7

S = S.replace('\n', '')
'''

計算が面倒くさいABC A問題