/* #region Head */

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

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pll = pair<ll, ll>;
template <class T> using vc = vector<T>;
template <class T> using vvc = vc<vc<T>>;
using vll = vc<ll>;
using vvll = vvc<ll>;
using vld = vc<ld>;
using vvld = vvc<ld>;
using vs = vc<string>;
using vvs = vvc<string>;
template <class T, class U> using um = unordered_map<T, U>;
template <class T> using pq = priority_queue<T>;
template <class T> using pqa = priority_queue<T, vc<T>, greater<T>>;
template <class T> using us = unordered_set<T>;

#define REP(i, m, n) for (ll i = (m), i##_len = (ll)(n); i < i##_len; ++(i))
#define REPM(i, m, n) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; ++(i))
#define REPR(i, m, n) for (ll i = (m), i##_min = (ll)(n); i >= i##_min; --(i))
#define REPD(i, m, n, d) for (ll i = (m), i##_len = (ll)(n); i < i##_len; i += (d))
#define REPMD(i, m, n, d) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; i += (d))
#define REPI(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define ALL(x) begin(x), end(x)
#define SIZE(x) ((ll)(x).size())
#define PERM(c)                                                                                                        \
    sort(ALL(c));                                                                                                      \
    for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))
#define UNIQ(v) v.erase(unique(ALL(v)), v.end());
#define CEIL(a, b) (((a) + (b)-1) / (b))

#define endl '\n'
#define sqrt sqrtl
#define floor floorl
#define log2 log2l

constexpr ll INF = 1'010'000'000'000'000'017LL;
constexpr int IINF = 1'000'000'007LL;
constexpr ll MOD = 1'000'000'007LL; // 1e9 + 7
// constexpr ll MOD = 998244353;
constexpr ld EPS = 1e-12;
constexpr ld PI = 3.14159265358979323846;

template <typename T> istream &operator>>(istream &is, vc<T> &vec) { // vector 入力
    for (T &x : vec) is >> x;
    return is;
}
template <typename T> ostream &operator<<(ostream &os, vc<T> &vec) { // vector 出力 (for dump)
    os << "{";
    REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "" : ", ");
    os << "}";
    return os;
}
template <typename T> ostream &operator>>(ostream &os, vc<T> &vec) { // vector 出力 (inline)
    REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "\n" : " ");
    return os;
}

template <typename T, typename U> istream &operator>>(istream &is, pair<T, U> &pair_var) { // pair 入力
    is >> pair_var.first >> pair_var.second;
    return is;
}
template <typename T, typename U> ostream &operator<<(ostream &os, pair<T, U> &pair_var) { // pair 出力
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

// map, um, set, us 出力
template <class T> ostream &out_iter(ostream &os, T &map_var) {
    os << "{";
    REPI(itr, map_var) {
        os << *itr;
        auto itrcp = itr;
        if (++itrcp != map_var.end()) os << ", ";
    }
    return os << "}";
}
template <typename T, typename U> ostream &operator<<(ostream &os, map<T, U> &map_var) { return out_iter(os, map_var); }
template <typename T, typename U> ostream &operator<<(ostream &os, um<T, U> &map_var) {
    os << "{";
    REPI(itr, map_var) {
        auto [key, value] = *itr;
        os << "(" << key << ", " << value << ")";
        auto itrcp = itr;
        if (++itrcp != map_var.end()) os << ", ";
    }
    os << "}";
    return os;
}
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) { return out_iter(os, set_var); }
template <typename T> ostream &operator<<(ostream &os, us<T> &set_var) { return out_iter(os, set_var); }
template <typename T> ostream &operator<<(ostream &os, pq<T> &pq_var) {
    pq<T> pq_cp(pq_var);
    os << "{";
    if (!pq_cp.empty()) {
        os << pq_cp.top(), pq_cp.pop();
        while (!pq_cp.empty()) os << ", " << pq_cp.top(), pq_cp.pop();
    }
    return os << "}";
}

void pprint() { cout << endl; }
template <class Head, class... Tail> void pprint(Head &&head, Tail &&... tail) {
    cout << head;
    if (sizeof...(Tail) > 0) cout << ' ';
    pprint(move(tail)...);
}

// dump
#define DUMPOUT cerr
void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) DUMPOUT << ", ";
    dump_func(move(tail)...);
}

// chmax (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>> bool chmax(T &xmax, const U &x, Comp comp = {}) {
    if (comp(xmax, x)) {
        xmax = x;
        return true;
    }
    return false;
}

// chmin (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>> bool chmin(T &xmin, const U &x, Comp comp = {}) {
    if (comp(x, xmin)) {
        xmin = x;
        return true;
    }
    return false;
}

// ローカル用
#define DEBUG_

#ifdef DEBUG_
#define DEB
#define dump(...)                                                                                                      \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                                                                    \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl                                        \
            << "    ",                                                                                                 \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

#define VAR(type, ...)                                                                                                 \
    type __VA_ARGS__;                                                                                                  \
    cin >> __VA_ARGS__;

template <typename T> istream &operator,(istream &is, T &rhs) { return is >> rhs; }
template <typename T> ostream &operator,(ostream &os, const T &rhs) { return os << ' ' << rhs; }

struct AtCoderInitialize {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;
    AtCoderInitialize() {
        ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
        cout << fixed << setprecision(IOS_PREC);
        if (AUTOFLUSH) cout << unitbuf;
    }
} ATCODER_INITIALIZE;

void Yn(bool p) { cout << (p ? "Yes" : "No") << endl; }
void YN(bool p) { cout << (p ? "YES" : "NO") << endl; }

/* #endregion */

// #include <atcoder/all>
// using namespace atcoder;

/* #region Graph */

// エッジ（本来エッジは双方向だが，ここでは単方向で管理）
template <class weight_t = int, class flow_t = int> struct Edge {
    ll src;          // エッジ始点となる頂点
    ll dst;          // エッジ終点となる頂点
    weight_t weight; // 重み
    flow_t cap;
    Edge() : src(0), dst(0), weight(0) {}
    Edge(ll src, ll dst, weight_t weight) : src(src), dst(dst), weight(weight) {}
    Edge(ll src, ll dst, weight_t weight, flow_t cap) : src(src), dst(dst), weight(weight), cap(cap) {}
    // Edge 標準出力
    friend ostream &operator<<(ostream &os, Edge &edge) {
        os << "(" << edge.src << " -> " << edge.dst << ", " << edge.weight << ")";
        return os;
    }
};
// 同じ頂点を始点とするエッジ集合
template <class weight_t = int, class flow_t = int> class Node : public vc<Edge<weight_t, flow_t>> {
  public:
    ll idx;
    Node() : vc<Edge<weight_t, flow_t>>() {}
    // void add(int a, int b, weight_t w, flow_t cap) { this->emplace_back(a, b, w, cap); };
};
// graph[i] := 頂点 i を始点とするエッジ集合
template <class weight_t = int, class flow_t = int> class Graph : public vc<Node<weight_t, flow_t>> {
  public:
    Graph() : vc<Node<weight_t, flow_t>>() {}
    Graph(int n) : vc<Node<weight_t, flow_t>>(n) { REP(i, 0, n)(*this)[i].idx = i; }
    // 単方向
    void add_arc(int a, int b, weight_t w = 1, flow_t cap = 1) { (*this)[a].emplace_back(a, b, w, cap); }
    // 双方向
    void add_edge(int a, int b, weight_t w = 1, flow_t cap = 1) { add_arc(a, b, w, cap), add_arc(b, a, w, cap); }
};
// using Array = vc<Weight>;
// using Matrix = vc<Array>;

/* #endregion */

template <class Weight = ll> struct Dijkstra {
    // pair 比較よりも struct 比較のほうが速い
    struct state {
        Weight cost;
        int dst;
        state(Weight cost, int dst) : cost(cost), dst(dst) {}
        bool operator<(const state &o) const { return cost > o.cost; }
        // bool operator>(const state &o) const { return cost > o.cost; }
    };

    Graph<Weight> graph;
    vc<Weight> dist;
    vc<int> bs;
    Weight inf;

    // コンストラクタ
    Dijkstra(int n, Weight inf) : graph(n), dist(n, inf), bs(n, -1), inf(inf) {}

    // 有向辺の追加
    void add_edge(int src, int dst, Weight cost) { graph.add_arc(src, dst, cost); }

    void build(int start, Weight init) {
        priority_queue<state> que; // 昇順に並べ替え，小さい順に取り出す
        fill(ALL(dist), inf);
        fill(ALL(bs), -1);
        dist[start] = init;
        que.emplace(init, start);
        while (que.size()) {
            state cur = que.top(); // tie(d, v) = que.top();
            que.pop();
            int cur_node = cur.dst;
            Weight cur_cost = cur.cost;

            if (dist[cur_node] < cur_cost) continue;
            for (Edge<Weight> edge : graph[cur_node])
                if (chmin(dist[edge.dst], dist[cur_node] + edge.weight)) {
                    que.emplace(dist[edge.dst], edge.dst);
                    bs[edge.dst] = cur_node;
                }
        }
    }

    // あるノードまでの距離を返す
    Weight operator[](int dst) { return dist[dst]; }

    // 経路復元
    vc<int> restore(int dst) {
        vc<int> res;
        if (bs[dst] < 0) return res;
        while (~dst) res.emplace_back(dst), dst = bs[dst];
        reverse(ALL(res));
        return res;
    }
};

// Problem
void solve() {
    VAR(ll, n, m);
    vll h(m), w(m), c(m);
    REP(i, 0, m) {
        cin >> h[i] >> w[i] >> c[i];
        --h[i], --w[i];
    }

    set<pll> sekisho;
    REP(i, 0, m) sekisho.emplace(h[i], w[i]);

    using Weight = ll;
    Dijkstra<Weight> dijkstra(n * n * 2, INF);
    ll offset = n * n;

    auto idx = [&](ll i, ll j) -> ll { return n * i + j; };
    auto idx2 = [&](ll i, ll j) -> ll { return n * i + j + offset; };

    REP(k, 0, m) {
        ll i = h[k], j = w[k];
        // dump(i, j);
        ll cur = idx(i, j);
        ll cur2 = idx2(i, j);
        if (j != n - 1) {
            dijkstra.add_edge(cur, idx(i, j + 1), c[k] + 1);   // 右
            dijkstra.add_edge(cur2, idx2(i, j + 1), c[k] + 1); // 右
            dijkstra.add_edge(cur, idx2(i, j + 1), 1);         // 右
        }
        if (j != 0) {
            dijkstra.add_edge(cur, idx(i, j - 1), c[k] + 1);   // 左
            dijkstra.add_edge(cur2, idx2(i, j - 1), c[k] + 1); // 左
            dijkstra.add_edge(cur, idx2(i, j - 1), 1);         // 左
        }
        if (i != n - 1) {
            dijkstra.add_edge(cur, idx(i + 1, j), c[k] + 1);   // 下
            dijkstra.add_edge(cur2, idx2(i + 1, j), c[k] + 1); // 下
            dijkstra.add_edge(cur, idx2(i + 1, j), 1);         // 下
        }
        if (i != 0) {
            dijkstra.add_edge(cur, idx(i - 1, j), c[k] + 1);   // 上
            dijkstra.add_edge(cur2, idx2(i - 1, j), c[k] + 1); // 上
            dijkstra.add_edge(cur, idx2(i - 1, j), 1);         // 上
        }
    }

    // 出るときにコストがかかるようにする
    REP(i, 0, n) REP(j, 0, n) {
        if (sekisho.count(pll(i, j))) continue;
        if (i == n - 1 && j == n - 1) continue;
        ll cur = idx(i, j);
        ll cur2 = idx2(i, j);
        if (j != n - 1) {
            dijkstra.add_edge(cur, idx(i, j + 1), 1);   // 右
            dijkstra.add_edge(cur2, idx2(i, j + 1), 1); // 右
        }
        if (j != 0) {
            dijkstra.add_edge(cur, idx(i, j - 1), 1);   // 左
            dijkstra.add_edge(cur2, idx2(i, j - 1), 1); // 左
        }
        if (i != n - 1) {
            dijkstra.add_edge(cur, idx(i + 1, j), 1);   // 下
            dijkstra.add_edge(cur2, idx2(i + 1, j), 1); // 下
        }
        if (i != 0) {
            dijkstra.add_edge(cur, idx(i - 1, j), 1);   // 上
            dijkstra.add_edge(cur2, idx2(i - 1, j), 1); // 上
        }
    }

    // dump(dijkstra.graph);
    dijkstra.build(0, 0);
    // dump(dijkstra.dist);
    ll ans = min(dijkstra.dist[idx(n - 1, n - 1)], dijkstra.dist[idx2(n - 1, n - 1)]);
    pprint(ans);
}

// entry point
int main() {
    solve();
    return 0;
}