#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;
 
#include <bits/stdc++.h>
using namespace std;

const ll INF=1LL<<60;

struct Edge{
  int to;
  ll w;
  Edge(int to,ll w) : to(to),w(w) {}
};

using Graph=vector<vector<Edge>>;
using pli=pair<ll,int>;

ll A[510][510];

template<class T> bool chmin(T& a,T b){
  if(a>b){
    a=b;
    return true;
  }
  return false;
}

int main(){
  int n,m;
  cin>>n>>m;
  
  Graph G(n*n);
  rep(i,m){
    int h,w;
    ll c;
    cin>>h>>w>>c;
    h--,w--;
    A[h][w]=c;
  }
  
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      if(i>0){
        G[(i-1)*n+j].push_back(Edge(i*n+j,A[i][j]+1));
        G[i*n+j].push_back(Edge((i-1)*n+j,A[i-1][j]+1));
      }
      if(i<n-1){
        G[(i+1)*n+j].push_back(Edge(i*n+j,A[i][j]+1));
        G[i*n+j].push_back(Edge((i+1)*n+j,A[i+1][j]+1));
     }
      if(j>0){
        G[i*n+j-1].push_back(Edge(i*n+j,A[i][j]+1));
        G[i*n+j].push_back(Edge(i*n+j-1,A[i][j-1]+1));
      }
      if(j<n-1){
        G[i*n+j+1].push_back(Edge(i*n+j,A[i][j]+1));
        G[i*n+j].push_back(Edge(i*n+j+1,A[i][j+1]+1));
      }
    }
  }
  
  vector<ll> dist(n*n,INF),dist1(n*n,INF);
  dist[0]=0,dist1[n*n-1]=0;
  
  priority_queue<pli,vector<pli>,greater<pli>> que,que1;
  que.push({dist[0],0});
  que1.push({dist1[n*n-1],n*n-1});
  
  while(!que.empty()){
    int v=que.top().second;
    ll d=que.top().first;
    que.pop();
    
    if(d>dist[v]) continue;
    
    for(auto e:G[v]){
      if(chmin(dist[e.to],dist[v]+e.w)){
        que.push({dist[e.to],e.to});
      }
    }
  }
  
  while(!que1.empty()){
    int v=que1.top().second;
    ll d=que1.top().first;
    que1.pop();
    
    if(d>dist1[v]) continue;
    
    for(auto e:G[v]){
      if(chmin(dist1[e.to],dist1[v]+e.w)){
        que1.push({dist1[e.to],e.to});
      }
    }
  }
  
  ll mi=INF;
  rep(i,n){
    rep(j,n){
      mi=min(mi,dist[i*n+j]+dist1[i*n+j]-2*A[i][j]);
    }
  }
  cout<<mi<<endl;
  
  return 0;
}