#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int(i) = 0; (i) < (n); (i)++)
#define FOR(i, m, n) for(int(i) = (m); (i) < (n); (i)++)
#define All(v) (v).begin(), (v).end()
#define pb push_back
#define MP(a, b) make_pair((a), (b))
template <class T> vector<T> make_vec(size_t a, T val) {
    return vector<T>(a, val);
}
template <class... Ts> auto make_vec(size_t a, Ts... ts) {
    return vector<decltype(make_vec(ts...))>(a, make_vec(ts...));
}
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using Graph = vector<vector<int>>;
template <typename T> struct edge {
    int to;
    T cost;
    edge(int t, T c) : to(t), cost(c) {}
};
template <typename T> using WGraph = vector<vector<edge<T>>>;
const int INF = 1 << 30;
const ll LINF = 1LL << 60;
const int MOD = 1e9 + 7;

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

int main() {
    int N, M;
    cin >> N >> M;
    auto dist = make_vec(N, N, 2, LINF);
    auto check = make_vec(N, N, 0LL);
    rep(i, M) {
        ll h, w, c;
        cin >> h >> w >> c;
        h--, w--;
        check[h][w] = c;
    }
    using State = pair<ll, pair<pii, int>>;
    priority_queue<State, vector<State>, greater<State>> que;
    que.push({0LL, {{0, 0}, 0}});
    dist[0][0][0] = 0;
    while(!que.empty()) {
        auto [cost, pp] = que.top();
        que.pop();
        auto [p, skipped] = pp;
        auto [x, y] = p;
        rep(i, 4) {
            int nx = x + dx[i], ny = y + dy[i];
            if(nx < 0 || nx >= N || ny < 0 || ny >= N)
                continue;
            if(skipped == 0) {
                if(check[ny][nx] && (dist[ny][nx][0] > cost + 1)) {
                    dist[ny][nx][1] = cost + 1 + check[ny][nx];
                    que.push({cost + 1, {{nx, ny}, 1}});
                }
            }
            if(dist[ny][nx][skipped] > cost + 1 + check[ny][nx]) {
                dist[ny][nx][skipped] = cost + 1 + check[ny][nx];
                que.push({cost + 1 + check[ny][nx], {{nx, ny}, skipped}});
            }
        }
    }
    cout << min(dist[N - 1][N - 1][1], dist[N - 1][N - 1][0]) << endl;
}