#include <bits/stdc++.h>
#define rep(i,n) for(int i=0; i<(int)(n); i++)

using namespace std;
using LL = long long;
const int Max_N = 510;
const LL INF = 1e15;

int N, M;
int dh[4]={1,0,-1,0};
int dw[4]={0,1,0,-1};
LL c[Max_N][Max_N];
LL dist[Max_N][Max_N];
LL ans=INF;
bool visited[Max_N][Max_N];

void dfs(int h, int w, LL max_f){
	if(h==N-1 and w==N-1){
		ans=min(ans,dist[h][w]-max_f);
		return;
	}
	visited[h][w]=true;
	rep(i,4){
		int nh=h+dh[i], nw=w+dw[i];
		if(nh<0 or nh>=N) continue;
		if(nw<0 or nw>=N) continue;
		if(visited[nh][nw]) continue;
		LL nf=dist[h][w]+c[nh][nw]+1;
		dist[nh][nw]=nf;
		LL max_nf=max(max_f,c[nh][nw]);
		dfs(nh,nw,max_nf);
	}
	visited[h][w]=false;
}

int main(){
	cin >> N >> M;
	rep(i,M){
		int h, w;
		LL f;
		cin >> h >> w >> f;
		h--; w--;
		c[h][w]=f;
	}
	dfs(0,0,0);
	cout << ans << endl;

	return 0;
}