#include<bits/stdc++.h>
using namespace std;
int n,m;
int vis[20][20];
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
int minn=0x7f,maxx=0;
inline void read(int& a){
	int s = 0, w = 1;
	char ch = getchar();
	while (ch < '0' || ch>'9'){
		if (ch == '-')
			w = -1;
			ch = getchar();
	}
	while (ch >= '0' && ch <= '9'){
		s = s * 10 + ch - '0';
		ch = getchar();
	}
	a = s * w;
}
void dfs(int x,int y,int t){
	//cout << x << ' ' << y << ' ' << t << endl;
	if (x==1&&y==n){
		if (t>maxx) maxx=t;
		if (t<minn) minn=t;
		//cout << maxx << ' ' << minn << endl; 
		return ;
	}
	for(int i=0;i<4;i++){
		int nx=x+dx[i];
		int ny=y+dy[i];
		if (vis[nx][ny]==0&&nx<=n&&nx>=1&&ny<=n&&ny>=1){
			vis[nx][ny]=1;
			dfs(nx,ny,t+1);
			vis[nx][ny]=0;
		}
	}
}
int main(){
	memset(vis,0,sizeof(vis));
	read(n);
	read(m);
	for(int i=1;i<=m;i++){
		int xx,yy;
		read(xx);
		read(yy);
		vis[xx][yy]=1;
	}
	if (m==0){
		cout << n+n-n*n;
	}
	vis[n][1]=1;
	dfs(n,1,0);
	cout << maxx-minn;
	return 0;
}

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