#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
#define all(a) (a).begin(),(a).end()
#define dunk(a) cout << (a) << endl
using namespace std;
typedef long long ll;

signed int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int x, y, x2, y2;
	cin >> x >> y >> x2 >> y2;

	x -= x2;
	y -= y2;

	ll ans = 1;

	for (int i = max(x, y) + 1; i <= (x + y); ++i) {
		ans *= i;
	}

	ll res = 1;
	for (int i = 1; i <= min(x, y); ++i) {
		res *= i;
	}

	ans /= res;

	dunk(ans);

	return 0;
}