#include <iostream>
using namespace std;

int main() {
	int a,b,c,d; cin>>a>>b>>c>>d;
	cout<<(a*d-b*c);
	return 0;
}

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = double;
const int mxn  = 1e6+10;
bool mt[20][20];
int h, w,a , b;
ll ans = 0;

void rec(int i=0, int j=0, int a0=0) {
	//cout << "[" << i << ", " << j << ", " << a0 << "]\n";
	if(a0>a or i>=h or j>=w) return;
	if(i==h-1 and j==w-1) {
		if(a==a0) {
			/*cout << "mat: \n";
			for(int x=0; x<h; x++) {
				for(int y=0; y<w; y++) cout << mt[x][y] <<  " "; cout << '\n';
			}*/
			ans++;
		}
		return;
	}
	if(!mt[i][j]) {
		if(j+1<w) rec(i, j+1, a0);
		else rec(i+1, 0, a0);

		if(j+1<w and !mt[i][j+1]) mt[i][j] = mt[i][j+1] = 1, rec(i, j+1, a0+1), mt[i][j] = mt[i][j+1] = 0;
		if(i+1<h and !mt[i+1][j]) {
			mt[i][j] = mt[i+1][j] = 1;
			if(j+1<w) rec(i, j+1, a0+1);
			else rec(i+1, 0, a0+1);
			mt[i][j] = mt[i+1][j] = 0;
		}
	}
	else {
		if(j+1<w) rec(i, j+1, a0);
		else rec(i+1, 0, a0);
	}
}

signed main() {
	ios_base::sync_with_stdio(0); cin.tie(0);

	cin>>h>>w>>a>>b;
	rec();
	cout << ans << '\n';

	return 0;
}