#include <iostream>
#include <algorithm>
using namespace std;
const int P = 1e9+7;
long long x, y, k, fact[1000005], inv[1000005], finv[1000005], p[1000005],a[1005];
int main()
{
    cin >> x >> y ;
    long long sum=0;
    for(int i=1;i<=y;i++)
    {
    	cin>>a[i];
    	sum+=a[i];
	}
    fact[0] = 1;
    for (int i = 1; i <= 1000000; i++)
        fact[i] = fact[i - 1] * i % P;
    finv[0] = finv[1] = inv[1] = 1;
    for (int i = 2; i <= 1000000; i++)
    {
        int k = P / i, r = P % i;
        inv[i] = ((-1 * k * inv[r] % P) + P) % P;
        finv[i] = finv[i - 1] * inv[i] % P;
    }
    x = x - sum + y;
    if (x < 0)
        cout << 0;
    else
        cout << fact[x - 1] * finv[y - 1] % P * finv[x - y] % P  << endl;
}