#include <iostream> #include <cstdio> using namespace std; typedef long long LL; LL c[13][13]; void init(){ for(int i = 0; i < 13; i++){ c[i][0] = 1; for(int j = 1; j < i; j++) c[i][j] = c[i-1][j]+c[i-1][j-1]; c[i][i] = 1; } } /** C[][] n!/(n1!*n2!*...*nk!) **/ int main() { // freopen("data.in", "r", stdin); int n, k, x; LL ans; init(); while(scanf("%d%d", &n, &k) != EOF){ ans = 1; while(k--){ scanf("%d", &x); ans *= c[n][x]; n-=x; } printf("%lld\n", ans); } return 0; }