1 #include<iostream>
 2 using namespace std;
 3 int n,m;
 4 int map[30][30];
 5 long long f[2][1<<20+1];
 6 int a[21];
 7 int main ()
 8 {
 9     cin>>n>>m;
10     for (int i=1,x,y;i<=m;i++) cin>>x>>y,map[x][y]=1;
11     for (int i=1;i<=n;i++)
12       for (int j=1;j<=n;j++)
13         a[i]=(a[i]<<1)+map[i][j];
14     int maxn=(1<<n)-1;
15     int x=1; 
16     for (int i=1;i<=n;i++) 
17      if (!(a[1]&(1<<i-1))) 
18       f[x][(1<<i-1)]=1;
19     for (int i=2;i<=n;i++)
20     {
21         x^=1;
22         for (int j=1;j<=maxn;j++)
23         {
24             if (__builtin_popcount(j)==i-1)
25             {
26                 for (int k=1;k<=n;k++)
27                 {
28                     if (!(j&(1<<k-1))&&!(a[i]&(1<<k-1)))
29                       f[x][j|(1<<k-1)]+=f[x^1][j];
30                 }
31             }
32         }
33     }
34     long long ans=0;
35     for (int i=1;i<=maxn;i++)
36       if (__builtin_popcount(i)==n)
37           ans+=f[x][i];
38     cout<<ans;
39 }