#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;
constexpr double PI = 3.14159265358979323846;
int dx[] = {0,0,1,-1};
int dy[] = {1,-1,0,0};

ll dp[505][505][2];
ll check[505][505];

int main(){
    int n,m;
    cin >> n >> m;
    rep(i,m){
        int h,w;
        ll c;
        cin >> h >> w >> c;
        --h;--w;
        check[h][w] = c;
    }
    rep(i,n)rep(j,n)rep(k,2) dp[i][j][k] = -1;
    priority_queue<pll,vector<pll>,greater<pll>> pq;
    auto id = [&](int i,int j,int k){ return 2*(i+j*n)+k;};
    pq.push(pll(0,id(0,0,0)));

    while(!pq.empty()){
        pll now = pq.top();
        pq.pop();
        ll res = now.first;
        int i = (now.second / 2) % n;
        int j = (now.second / 2) / n;
        int k = (now.second % 2);
        if(dp[i][j][k] != -1) continue;

        dp[i][j][k] = res;
        rep(c,4){
            int now_i = i + dx[c];
            int now_j = j + dy[c];
            if(now_i < 0 || now_j < 0 || n <= now_i || n <= now_j) continue;
            if(check[now_i][now_j] == 0){
                if(dp[now_i][now_j][k] == -1) pq.push(pll(res+1,id(now_i,now_j,k)));
            }else{
                if(k == 0){
                    if(dp[now_i][now_j][k] == -1) pq.push(pll(res+1+check[now_i][now_j],id(now_i,now_j,k)));
                    if(dp[now_i][now_j][k+1] == -1) pq.push(pll(res+1,id(now_i,now_j,k+1)));
                }else{
                    if(dp[now_i][now_j][k] == -1) pq.push(pll(res+1+check[now_i][now_j],id(now_i,now_j,k)));
                }
            }
        }
    }

    cout << min(dp[n-1][n-1][0],dp[n-1][n-1][1]) << endl;
    return 0;
}