#include <bits/stdc++.h>

using namespace std;
using ll = long long;

template<typename T> using pq = priority_queue<T, vector<T>, greater<T>>;

int main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    ll N, M, h, w, x, nh, nw, d, alt;
    bool f;
    cin >> N >> M;
    vector c(N, vector<ll>(N));
    for (int i=0; i<M; i++){
        cin >> h >> w >> x;
        h--; w--;
        c[h][w] = x;
    }

    pq<tuple<ll, int, int, bool>> que;
    int dx[4] = {1, 0, -1, 0};
    int dy[4] = {0, 1, 0, -1};
    vector dist(N, vector(N, vector<ll>(2, 1e18)));
    vector vst(N, vector(N, vector<bool>(2, 0)));

    dist[0][0][0] = 0;
    que.push({0, 0, 0, 0});

    while(!que.empty()){
        tie(d, h, w, f) = que.top();
        que.pop();
        if (vst[h][w][f]) continue;
        vst[h][w][f] = 1;
        for (int dir=0; dir<4; dir++){
            nh = h + dx[dir];
            nw = w + dy[dir];
            if (nh < 0 || nh >= N || nw < 0 || nw >= N) continue;
            alt = dist[h][w][f]+1+c[nh][nw];
            if (alt < dist[nh][nw][f]){
                dist[nh][nw][f] = alt;
                que.push({dist[nh][nw][f], nh, nw, f});
            }
            if (f == 0){
                alt = dist[h][w][f]+1;
                if (alt < dist[nh][nw][1]){
                    dist[nh][nw][1] = alt;
                    que.push({dist[nh][nw][1], nh, nw, 1});
                }
            }
        }
    }

    cout << min(dist[N-1][N-1][0], dist[N-1][N-1][1]) << endl;

    return 0;
}