#include <stdio.h>
#include <vector>
#include <queue>
#include <stack>
#include <list>
#include <set>
#include <algorithm>
#include <utility>
#include <string>
#include <functional>
#include <sstream>
#include <fstream>
using namespace std;
#define FOR(i,a,b) for (typeof a i=a;i<=b;i++)
#define fori(it,v) for (typeof ((v).begin()) it=(v).begin();it!=(v).end();it++)
#define pb push_back
#define mp make_pair
#define fs first
#define ss second
#define all(c) c.begin(),c.end()
#define pf push_front
#define popb pop_back
#define popf pop_front
ofstream fout;
ostringstream out;
istringstream in;
void initializare_citire()
{
	ifstream fin;
	fin.open("gropi.in",ifstream::in);
	fout.open("gropi.out",ofstream::out);
	string aux;
	getline(fin,aux,'\0');
	in.str(aux);
	fin.close();
}
void scriere()
{
	fout<<out.str();
	fout.close();
}
vector<bool> ver;
vector<int> a;
int main()
{
	int n,m,x;
	initializare_citire();
	in>>n>>m;
	a.resize(n+3);
	ver.resize(n+3,0);
	FOR(i,1,m)
	{
		in>>x;
		ver[x]=1;
	}
	a[1]=1;
	FOR(i,2,n)
	{
		if (ver[i])
			a[i]=a[i-1];
		else
			a[i]=a[i-1]*(i-1);
	}
	out<<a[n];
	scriere();
	return 0;
}