Problem:
Score : 100 points

Problem Statement

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.

Sample Input 1

3 2
2 1

Sample Output 1

1

There are 3 rows and 2 columns of cells. When two rows and one column are chosen and painted in black, there is always one white cell that remains.

Sample Input 2

5 5
2 3

Sample Output 2

6

Sample Input 3

2 4
2 4

Sample Output 3

0

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#pragma region includes, macros
#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <vector>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <map>
#include <set>
#include <list>
#include <unordered_set>
#include <unordered_map>
#include <stdlib.h>

using namespace std;
 
typedef long long i64;
typedef pair<i64, i64> P;

template<class T>
const T INF = numeric_limits<T>::max();
template<class T>
const T SINF = numeric_limits<T>::max() / 10;
static const i64 MOD = 1000000007;

//int dx[4] = {0,1,0,-1}, dy[4] = {-1,0,1,0};
//int dx[5] = {-1,0,0,0,1}, dy[5] = {0,-1,0,1,0};
//int dx[8] = {-1,0,1,1,1,0,-1,-1}, dy[8] = {1,1,1,0,-1,-1,-1,0};
//int dx[9] = {-1,0,1,1,1,0,-1,-1,0}, dy[9] = {1,1,1,0,-1,-1,-1,0,0};

struct edge {
	i64 from, to, cost;
	edge(i64 to, i64 cost) : from(-1), to(to), cost(cost) {}
	edge(i64 src, i64 to, i64 cost) : from(src), to(to), cost(cost) {}
};

template<typename T>
vector<T> make_v(size_t a){return vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
	return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

template<typename T,typename V>
typename enable_if<is_class<T>::value==0>::type
fill_v(T &t,const V &v){t=v;}

template<typename T,typename V>
typename enable_if<is_class<T>::value!=0>::type
fill_v(T &t,const V &v){
	for(auto &e:t) fill_v(e,v);
}
#pragma endregion

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	int H, W, h, w;
	cin >> H >> W >> h >> w;
	cout << (H - h) * (W - w) << endl;
}