#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL_
void debug_out() {cerr << endl;}
template<typename Head,typename... Tail> void debug_out(Head H,Tail... T){cerr << ' ' << H; debug_out(T...);}
#define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:",debug_out(__VA_ARGS__)
#define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl;
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
#define rep(i,n) for (int i = 0; i < (int)(n); i ++)
#define irep(i,n) for (int i = (int)(n) - 1;i >= 0;--i)
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;// = 998244353;
constexpr double EPS = 1e-4;
constexpr double PI = 3.14159265358979;

const int dy[4] = {-1,0,1,0};
const int dx[4] = {0,-1,0,1};

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

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

    int M = N*N;
    vector<ll> dist(2*M,HINF);
    dist[0] = 0;

    priority_queue<PL,vector<PL>,greater<P>> q;
    q.push({0,0});

    while (!q.empty()) {
        PL p = q.top(); q.pop();
        ll d = p.first;
        int v = p.second;
        if (dist[v] < d) continue;
        if (v >= M) {
            int ny = (v - M)/N;
            int nx = (v - M)%N;
            rep(k,4) {
                if (ny + dy[k] < 0 || ny + dy[k] >= N || nx + dx[k] < 0 || nx + dx[k] >= N) continue;
                int y = ny + dy[k];
                int x = nx + dx[k];
                if (dist[y*N + x + M] > dist[ny*N + nx + M] + penalty[y][x] + 1) {
                    dist[y*N + x + M] = dist[ny*N + nx + M] + penalty[y][x] + 1;
                    q.push({dist[y*N + x + M],y*N + x + M});
                }
            }
        } else {
            int ny = v/N;
            int nx = v%N;
            rep(k,4) {
                if (ny + dy[k] < 0 || ny + dy[k] >= N || nx + dx[k] < 0 || nx + dx[k] >= N) continue;
                int y = ny + dy[k];
                int x = nx + dx[k];
                if (dist[y*N + x] > dist[ny*N + nx] + penalty[y][x] + 1) {
                    dist[y*N + x] = dist[ny*N + nx] + penalty[y][x] + 1;
                    q.push({dist[y*N + x],y*N + x});
                }
                if (dist[y*N +x + M] > dist[ny*N + nx] + 1) {
                    dist[y*N + x + M] = dist[ny*N + nx] + 1;
                    q.push({dist[y*N + x + M],y*N + x + M});
                }
            }
        }
    }
    
    cout << dist.back() << '\n';
    return 0;
}