#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

const ll INF = 1e+17;

typedef pair<int, vector<int>> P;

struct edge{
    int to;
    ll cost;
};

int H, W;

int dh[4] = {0, 0, -1, 1};
int dw[4] = {-1, 1, 0, 0};

bool is_in_field(int i, int j){
    return i >= 0 && i < H && j >= 0 && j < W;
}

ll dist[500][500][2];
ll cost[500][500];

void input(){
    int M;
    cin >> H >> M;
    W = H;
    for(int i = 0; i < M; i++) {
        int h, w; cin >> h >> w; h--; w--;
        ll c; cin >> c;
        cost[h][w] = c;
    }    
}

void dijkstra(){
    priority_queue<P, vector<P>, greater<P>> que;
    for(int i = 0; i < H; i++){
        for(int j = 0; j < W; j++){
            dist[i][j][0] = INF;
            dist[i][j][1] = INF;
        }
    }
    dist[0][0][0] = 0;
    vector<int> v = {0, 0, 0};
    que.push(P(0, v));
    while(!que.empty()){
        P p = que.top();que.pop();
        auto v = p.second;
        int h = v[0];
        int w = v[1];
        int s = v[2];
        ll d = p.first;
        if(dist[h][w][s] < d) continue;
        for(int i = 0; i < 4; i++){
            int h_to = h + dh[i];
            int w_to = w + dw[i];
            if(!is_in_field(h_to, w_to)) continue;
            if(d+cost[h_to][w_to]+1 < dist[h_to][w_to][s]){
                dist[h_to][w_to][s] = d+cost[h_to][w_to]+1;
                vector<int> v = {h_to, w_to, s};
                que.push(P(dist[h_to][w_to][s], v));
            }
            if(s == 0 && d+1 < dist[h_to][w_to][1]){
                dist[h_to][w_to][1] = d+1;
                vector<int> v = {h_to, w_to, 1};
                que.push(P(dist[h_to][w_to][1], v));
            }
        }
    }
}

void solve(){
    dijkstra();
    cout << min(dist[H-1][W-1][0], dist[H-1][W-1][1]) << endl;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    input();
    solve();
}