#include <bits/stdc++.h>

using namespace std;
ifstream in("grigo.in");
ofstream out("grigo.out");
const int nmax=100000, mod=1000003;
int n, m, v[nmax+1], f[nmax+1];
int main()
{
    in>>n>>m;
    for(int i=1; i<=m; i++)
    {
        in>>v[i];
        f[v[i]]++;
    }
    //sort(v+1, v+m+1);
    long long rez=1;
    for(int i=2; i<=n; i++)
    {
        if(f[i]==1)//vizibil-o singura posibiliate
        {
            rez=rez*1;
        }
        else//invizibil-poate lua orice valoare de la 1 la i-1
        {
            rez=rez*(i-1);
        }

    }
    out<<rez;
}