关于求组合数

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;

#define int long long
#define endl '\n'
#define pii pair<int,int>
#define fr first
#define sd second
#define mkpr make_pair
#define debug puts("114514")

const int mod = 1e9 + 7;

int jcn = 1, jcm = 1, jcn_m = 1;

int quick_pow(int a, int n) {
	if (a == 0) return 0;
	if (n == 0) return 1;
	if (n == 1) return a % mod;
	int t = quick_pow(a, n >> 1) % mod;
	return t * t % mod * quick_pow(a, n & 1) % mod;
}
// 费小求逆元
signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
//	freopen("test.txt", "r", stdin);
	int n, m;
	cin >> n >> m;
	for (int i = 2; i <= n; i++) {
		jcn *= i;
		jcn %= mod;
	}
	for (int i = 2; i <= n - m; i++) {
		jcn_m *= i;
		jcn_m %= mod;
	}
	for (int i = 2; i <= m; i++) {
		jcm *= i;
		jcm %= mod;
	}
	cout << (jcn * quick_pow(jcn_m, mod - 2) % mod * quick_pow(jcm, mod - 2) % mod);
//	cout << quick_pow(6, mod - 2);
}