#include<stdio.h>typedef long long ll;ll C(int n, int k){    ll ans = 1;    int temp = 0;    while(temp != k) { ans *= n - temp; temp++; }    for(int i=1; i<=k; i++) ans /= i;    return ans;}int main(){    int ex, m;    while(scanf("%d%d", &ex, &m) == 2) {        int array[15];        for(int i=0; i<m; i++) scanf("%d", &array[i]);        ll res = 1; int now = ex;        for(int i=0; i<m; i++) if(array[i]) {            res *= C(now, array[i]);            now -= array[i];        }        printf("%lld\n", res);    }    return 0;}