#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
using namespace std;

using ll = long long;
using P = pair<int, int>;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

constexpr ll inf = 1e+17;
using Data = tuple<ll, int, int, int>;
ll dist[505][505][2];
constexpr int dx[4] = {1, 0, -1, 0};
constexpr int dy[4] = {0, 1, 0, -1};

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

    int N, M;
    cin >> N >> M;
    vector<vector<int>> c(N, vector<int>(N));
    for(int i = 0; i < M; ++i){
        int h, w;
        cin >> h >> w;
        --h, --w;
        cin >> c[h][w];
    }
    for(int i = 0; i < N; ++i){
        for(int j = 0; j < N; ++j){
            dist[i][j][0] = dist[i][j][1] = inf;
        }
    }
    dist[0][0][0] = 0;
    priority_queue<Data, vector<Data>, greater<Data>> que;
    que.emplace(0, 0, 0, 0);
    while(!que.empty()){
        auto [d, x, y , f] = que.top();
        que.pop();
        if(d > dist[x][y][f])   continue;
        for(int i = 0; i < 4; ++i){
            int nx = x + dx[i], ny = y + dy[i];
            if(nx < 0 || nx >= N || ny < 0 || ny >= N)  continue;
            if(f == 1){
                if(chmin(dist[nx][ny][1], dist[x][y][1] + 1 + c[nx][ny])){
                    que.emplace(dist[nx][ny][1], nx, ny, 1);
                }
            }
            else{
                if(chmin(dist[nx][ny][1], dist[x][y][0] + 1)){
                    que.emplace(dist[nx][ny][1], nx, ny, 1);
                }
                if(chmin(dist[nx][ny][0], dist[x][y][0] + 1 + c[nx][ny])){
                    que.emplace(dist[nx][ny][0], nx, ny, 0);
                }
            }
        }
    }
    cout << min(dist[N - 1][N - 1][0], dist[N - 1][N - 1][1]) << endl;

    return 0;
}