#include <cstdio>
#include <cstdlib>
#include <set>
#include <algorithm>
using namespace std;
const int MAXN=21;
int N,M;
int Ans=0;
set<int>Fuka[MAXN];
int Pos[MAXN];

bool Check(int k,int j)
{
	for(int i=1;i<=N;i++)
	{
		if(Pos[i]==j) return false;
	}
	if(Fuka[k].find(j)!=Fuka[k].end()) return false;
	return true;
}

int main()
{
	freopen("examone.in","r",stdin);
	freopen("examone.out","w",stdout);
	scanf("%d %d\n",&N,&M);
	int a,b;
	for(int i=1;i<=M;i++)
	{
		scanf("%d %d\n",&a,&b);
		Fuka[a].insert(b);
	}
	for(int i=1;i<=N;i++) Pos[i]=0;
	int i=1,j=1;
	while(i<=N)
	{
		while(j<=N)
		{
			if(Check(i,j))
			{
				Pos[i]=j;
				j=1;
				break;
			}
			else j++;
		}
		if(Pos[i]==0)
		{
			if(i==1)
			{
				printf("%d\n",Ans);
				exit(0);
			}
			else
			{
				i--;
				j=Pos[i]+1;
				Pos[i]=0;
				continue;
			}
		}
		if(i==N)
		{
			Ans++;
			j=Pos[i]+1;
			Pos[i]=0;
			continue;
		}
		i++;
	}
	return 0;
}