#include<iostream>
#include<array>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<complex>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<tuple>
#include<cassert>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define Per(i,sta,n) for(int i=n-1;i>=sta;i--)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef long double ld;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;
typedef pair<P, int> PP;
typedef pair<ll, PP> node;
int dx[4]={1,-1,0,0};
int dy[4]={0,0,1,-1};
template<class T>bool chmax(T &a, const T &b) {if(a<b){a=b;return 1;}return 0;}
template<class T>bool chmin(T &a, const T &b) {if(b<a){a=b;return 1;}return 0;}

int n,m;
ll cost[510][510];
ll dist[510][510][2];

void solve(){
    cin >> n >> m;
    rep(i,m){
        int h,w;ll c;
        cin >> h >> w >> c;h--;w--;
        cost[h][w]=c;
    }
    rep(i,n){
        rep(j,n){
            rep(flag,2){
                dist[i][j][flag]=INF;
            }
        }
    }
    priority_queue<node,vector<node>,greater<node>> que;
    que.push(node(0,PP(P(0,0),0)));
    while(!que.empty()){
        node nod=que.top();que.pop();
        ll d=nod.first;int y=nod.second.first.first,x=nod.second.first.second,flag=nod.second.second;
        //cout << y << " " << x << " " << flag << " : " << d << endl;
        rep(k,4){
            int ny=y+dy[k],nx=x+dx[k];
            if(ny<0 || ny>n-1 || nx<0 || nx>n-1) continue;
            //cout << ny << " " << nx << endl;
            if(flag){
                if(chmin(dist[ny][nx][1],d+1+cost[ny][nx])){
                    que.push(node(dist[ny][nx][1],PP(P(ny,nx),1)));
                }
            }
            else{
                if(chmin(dist[ny][nx][1],d+1)){
                    que.push(node(dist[ny][nx][1],PP(P(ny,nx),1)));
                }
                if(chmin(dist[ny][nx][0],d+1+cost[ny][nx])){
                    que.push(node(dist[ny][nx][0],PP(P(ny,nx),0)));
                }
            }
        }
    }
    cout << min(dist[n-1][n-1][0],dist[n-1][n-1][1]) << endl;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(50);
    solve();
}