D.cpp

#include <iostream>
#include <algorithm>
using namespace std;
 
const long long mod = 1e9 + 7ll;
const int N = 2 * 1e5 + 10;
 
long long fac[N];
 
long long pw(long long a, long long b){
	if ( b == 0ll ) return 1ll;
	long long r = pw(a, b/2ll);
	r = (r * r) % mod;
	if (b % 2ll) r = (a * r) % mod;
	return r;
}

long long inv(long long x){
	return pw(x, mod - 2ll);
}

long long path(long long a, long long b) {
	long long ans = fac[a + b];
	ans = (ans * inv(fac[a])) % mod;
	ans = (ans * inv(fac[b])) % mod;
	return ans;
}
int main () {
 
	int h, w, a, b;
	cin >> h >> w >> a >> b;
	fac[0] = 1ll;
	for ( int i=1 ; i<N ; i++){
		fac[i] = (i * fac[i-1]) % mod;
	}
	long long ans = 0ll;
	for (int i=h-a, j=b+1 ; i>0 && j<=w ; i--, j++) {
		ans += (path(i-1, j-1) * path(h-i, w-j)) % mod;
	}
 
	ans = (ans % mod + mod) % mod;
	cout << ans;
	return 0;
}