#include<bits/stdc++.h>
using namespace std;
int b[105][105],sum,m,cnt,minn=-1;
int dx[]={0,1,0,-1},dy[]={1,0,-1,0};
bool bo[105][105];
void dfs(int x,int y){
	if(x==m&&y==m){
		if(minn==-1||sum<minn)minn=sum;
		return;
	}
	if(x<1||y<1||x>m||y>m || cnt>1 || bo[x][y])return;
	bo[x][y]=true; 
	for(int i=0;i<4;i++){
		int dsum=0,now=b[x][y],next=b[x+dx[i]][y+dy[i]],temp=cnt;
		if(next==-1){
			cnt++,dsum=2,b[x+dx[i]][y+dy[i]]=now;
		}else{
			if(next!=now)dsum=1;
			cnt=0;
		}
		sum+=dsum;
		dfs(x+dx[i],y+dy[i]);
		sum-=dsum;
		b[x+dx[i]][y+dy[i]]=next,cnt=temp;
	}
	bo[x][y]=false;
}
int main(){
	memset(b,-1,sizeof(b));
	int x,y,c,n;
	cin>>m>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>x>>y>>c;
		b[x][y]=c;
	}
	dfs(1,1);
	cout<<minn;
	return 0;
}