#include <iostream>
#include <fstream>
#define vv 100001

using namespace std;

ifstream fin ("grigo.in");
ofstream fout ("grigo.out");

int n,m,x,a[vv];

void citire()
{
    fin >> n >> m;
    for (int i=1; i<=m; i++)
    {
        fin >> x;
        a[x]=1;
    }
}

int main()
{
    citire();

    x=1;
    for (int i=2; i<=n-m+1; i++)
        x*=i;
    fout << x <<"\n";

    return 0;
}