#include <iostream>
using namespace std;
int main() {
	int a,n,m,mx,my;
	cin >> n;
	cin >> m;
	cin >> mx;
	cin >> my;
	a = n*m + 1;
	if (m >= mx + 1 >= 0 & n >= my + 2 >= 0) {
		a -= (mx + 1)*(my + 2) + 1;
		cout << (mx + 1)*(my + 2) + 1 << endl;
	}
	else {
		if (m >= mx - 2 >= 0 & n >= my - 1 >= 0) {
			a -= (mx - 2)*(my - 1) + 1;
			cout << (mx - 2)*(my - 1) + 1 << endl;
		}
		if (m >= mx - 1 >= 0 & n >= my - 2 >= 0) {
			a -= (mx - 1)*(my - 2) + 1;
			cout << (mx - 1)*(my - 2) + 1 << endl;
		}
		if (m >= mx + 2 >= 0 & n >= my - 1 >= 0);
		else {
			if (m >= mx + 1 >= 0 & n >= my - 2 >= 0) {
				a -= (mx + 1)*(my - 2) + 1;
				cout << (mx + 1)*(my - 2) + 1 << endl;
			}
		}
		if (m >= mx - 1 >= 0 & n >= my + 2 >= 0) {
			a -= (mx - 1)*(my + 2) + 1;
			cout << (mx - 1)*(my + 2) + 1 << endl;
		}
		else {
			if (m >= mx - 2 >= 0 & n >= my + 1 >= 0) {
				a -= (mx - 2)*(my + 1) + 1;
				cout << (mx - 2)*(my + 1) + 1 << endl;
			}
		}
	}
	if (m >= mx + 2 >= 0 & n >= my + 1 >= 0) {
		a -= (mx + 2)*(my + 1) + 1;
		cout << (mx + 2)*(my + 1) + 1 << endl;
	}
	else {
		if (m >= mx - 2 >= 0 & n >= my - 1 >= 0) {
			a -= (mx - 2)*(my - 1) + 1;
			cout << (mx - 2)*(my - 1) + 1 << endl;
		}
		if (m >= mx - 1 >= 0 & n >= my + 2 >= 0) {
		}
		else {
			if (m >= mx - 2 >= 0 & n >= my + 1 >= 0) {
				a -= (mx - 2)*(my + 1) + 1;
				cout << (mx - 2)*(my + 1) + 1 << endl;
			}
		}
		if (m >= mx + 2 >= 0 & n >= my - 1 >= 0) {
			a -= (mx + 2)*(my - 1) + 1;
			cout << (mx + 2)*(my - 1) + 1;
		}
		else {
			if (m >= mx + 1 >= 0 & n >= my - 2 >= 0) {
				a -= (mx + 1)*(my - 2) + 1;
				cout << (mx + 1)*(my - 2) + 1 << endl;
			}
		}
		if (m >= mx - 1 >= 0 & n >= my - 2 >= 0) {
			a -= (mx - 1)*(my - 2) + 1;
			cout << (mx - 1)*(my - 2) + 1 << endl;
		}
	}
	
	
	
	
	cout << a;
	return 0;
	system("pause");
}