#include<bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define all(a) a.begin(),a.end()
#define rall(c) (c).rbegin(),(c).rend()
#define mp make_pair
#define endl '\n'
#define vec vector<ll>
#define mat vector<vector<ll> >
#define fi first
#define se second
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
typedef long double ld;
typedef complex<double> comp;
const ll INF=1e9+7;
const ll inf=INF*INF;
const ll MOD=1e9+7;
const ll mod=MOD;
const int MAX=1000010;
const double PI=acos(-1.0);

//dijkstra(O(ElogV))
//0-indexed
void dijkstra(ll n,ll s,vector<vector<pll> >&G,vector<ll> &d){
    priority_queue<pll,vector<pll>,greater<pll> >pq;
    rep(i,n){
        d[i]=inf;
    }
    d[s]=0;
    pq.push(mp(0,s));
    while(!pq.empty()){
        pll k=pq.top();pq.pop();
        ll v=k.second;
        if(d[v]<k.first)continue;
        for(auto e:G[v]){
            if(d[e.first]>d[v]+e.second){
                d[e.first]=d[v]+e.second;
                pq.push(mp(d[e.first],e.first));
            }
        }
    }
}

ll dx[]={0,1,0,-1};
ll dy[]={1,0,-1,0};

signed main(){
    ll n,m;cin>>n>>m;
    vector<vector<ll> >c(n,vector<ll>(n));
    rep(i,m){
        ll h,w;cin>>h>>w;
        h--;w--;
        cin>>c[h][w];
    }
    vector<vector<pll> >G(n*n);
    vector<vector<pll> >G2(n*n);
    vector<ll>d(n*n);
    vector<ll>d2(n*n);
    rep(i,n){
        rep(j,n){
            ll now=i*n+j;
            rep(l,4){
                ll nx=i+dx[l],ny=j+dy[l];
                if(nx<0||nx>=n||ny<0||ny>=n)continue;
                ll cost=1+c[nx][ny];
                ll to=nx*n+ny;
                G[now].pb(mp(to,cost));
                G2[to].pb(mp(now,cost));
            }
        }
    }
    dijkstra(n*n,0,G,d);
    dijkstra(n*n,n*n-1,G2,d2);
    ll ans=inf;
    REP(i,1,n*n-1){
        ll h=i/n;
        ll w=i%n;
        ll dd=d[i]+d2[i]-c[h][w];
        if(c[h][w]==0)dd--;
        ans=min(d[i]+d2[i]-c[h][w],ans);
    }
    cout<<ans<<endl;
}