#include<stdio.h>
char v[100000];
int main()
{
    FILE *fin,*fout;
    fin=fopen("grigo.in","r");
    fout=fopen("grigo.out","w");
    int n,m;
    fscanf(fin,"%d%d",&n,&m);
    int i;
    for(i=0;i<m;i++)
    {
        int x;
        fscanf(fin,"%d",&x);
        v[x]=1;
    }
    int sol=1;
    for(i=2;i<=n;i++)
        if(v[i]==0)
            sol*=i-1;
    fprintf(fout,"%d",sol);
    return 0;
}