# AtCoder Regular Contest 058 D - いろはちゃんとマス目 / Iroha and a Grid

,

## implementation

#include <cstdio>
#include <vector>
#include <cassert>
#define repeat(i,n) for (int i = 0; (i) < (n); ++(i))
#define repeat_from(i,m,n) for (int i = (m); (i) < (n); ++(i))
typedef long long ll;
using namespace std;

const int mod = 1e9+7;

ll powi(ll x, ll y, ll p) {
assert (y >= 0);
x = (x % p + p) % p;
ll z = 1;
for (ll i = 1; i <= y; i <<= 1) {
if (y & i) z = z * x % p;
x = x * x % p;
}
return z;
}
ll inv(ll x, ll p) {
assert ((x % p + p) % p != 0);
return powi(x, p-2, p);
}
ll choose(ll n, ll r) { // O(n) at first time, otherwise O(1)
static vector<ll> fact(1,1);
static vector<ll> ifact(1,1);
if (fact.size() <= n) {
int l = fact.size();
fact.resize( n + 1);
ifact.resize(n + 1);
repeat_from (i,l,n+1) {
fact[i]  = fact[i-1] * i % mod;
ifact[i] = inv(fact[i], mod);
}
}
r = min(r, n - r);
return fact[n] * ifact[n-r] % mod * ifact[r] % mod;
}

int main() {
int h, w, a, b; scanf("%d%d%d%d", &h, &w, &a, &b);
ll ans = 0;
a = h - a - 1;
for (; a >= 0 and b < w; -- a, ++ b) {
int c = h - a - 1;
int d = w - b - 1;
ans += choose(a + b, a) * choose(c + d, d) % mod;
ans %= mod;
}
printf("%lld\n", ans);
return 0;
}