#include <bits/stdc++.h>
using namespace std;

int dx[] = {0,1,0,-1};
int dy[] = {1,0,-1,0};
int cost[505][505];
long long dist[505][505][5];
vector<array<long long,3>> road[505][505];

int main() {
    int N,M;
    cin >> N >> M;
    for(int i = 0; i < N; i++) {
        for(int j = 0; j < N; j++) {
            cost[i][j] = 1;
            for(int k = 0; k < 2; k++) {
                dist[i][j][k] = 1e18;
            }
        }
    }
    dist[0][0][0] = 0;
    for(int i = 0; i < M; i++) {
        int h,w,c;
        cin >> h >> w >> c;
        h--;
        w--;
        cost[h][w] += c;
    }
    priority_queue<array<long long,4>,vector<array<long long,4>>,greater<array<long long,4>>>que;
    que.push({0,0,0,0});
    while(!que.empty()) {
        array<long long,4> x = que.top();
        que.pop();
        if(x[0] > dist[x[1]][x[2]][x[3]]) {
            continue;
        }
        for(int i = 0; i < 4; i++) {
            int nx = x[1]+dx[i];
            int ny = x[2]+dy[i];
            if(nx >= 0 && nx < N && ny >= 0 && ny < N) {
                if(x[3] == 0) {
                    if(dist[nx][ny][0] > x[0]+cost[nx][ny]) {
                        dist[nx][ny][0] = x[0]+cost[nx][ny];
                        que.push({dist[nx][ny][0],nx,ny,0});
                    }
                    if(dist[nx][ny][1] > x[0]+1) {
                        dist[nx][ny][1] = x[0]+1;
                        que.push({dist[nx][ny][1],nx,ny,1});
                    }
                }
                else {
                    if(dist[nx][ny][1] > x[0]+cost[nx][ny]) {
                        dist[nx][ny][1] = x[0]+cost[nx][ny];
                        que.push({dist[nx][ny][1],nx,ny,1});
                    }
                }
            }
        }
    }
    cout << min(dist[N-1][N-1][0],dist[N-1][N-1][1]) << endl;
}