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

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

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    int N,M; cin >> N >> M;
    vector<vector<int>> A(N, vector<int> (N,0));
    rep(i,M){
        int h,w,c; cin >> h >> w >> c;
        h--; w--;
        A[h][w] = c;
    }

    priority_queue<tuple<ll,int,int,int>, vector<tuple<ll,int,int,int>>,greater<tuple<ll,int,int,int>>> Q;
    vector<vector<vector<ll>>> cost(N,vector<vector<ll>>(N, vector<ll> (2,-1)));
    Q.push({0,0,0,0});

    while(!Q.empty()){
        auto [sum,y,x,u] = Q.top(); Q.pop();

        if(cost[y][x][u] == -1){
            cost[y][x][u] = sum;

            rep(i,4){
                int ny = y + dy[i];
                int nx = x + dx[i];
                if(ny < 0 || N <= ny || nx < 0 || N <= nx) continue;
                if(cost[ny][nx][u] == -1){
                    Q.push({sum+A[ny][nx]+1, ny, nx, u});
                }
                if(u == 0 && cost[ny][nx][1] == -1){
                    Q.push({sum+1, ny, nx, 1});
                }
            }
        }
    }

    cout << cost[N-1][N-1][1] << '\n';
}