// Template
#include "bits/stdc++.h"
#define rep_override(x, y, z, name, ...) name
#define rep2(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep3(i, l, r) for (int i = (int)(l); i < (int)(r); ++i)
#define rep(...) rep_override(__VA_ARGS__, rep3, rep2)(__VA_ARGS__)
#define per(i, n) for (int i = (int)(n) - 1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
using namespace std;
using ll = long long;
constexpr int inf = 1001001001;
constexpr ll INF = 3003003003003003003LL;
template <typename T>
inline bool chmin(T &x, const T &y) {
    if (x > y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T> 
inline bool chmax(T &x, const T &y) {
    if (x < y) {
        x = y;
        return true;
    }
    return false;
}
struct IOSET {
    IOSET() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(10);
    }
} ioset;
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &element : vec) is >> element;
    return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
    for (int i = 0, vec_len = (int)vec.size(); i < vec_len; ++i) {
        os << vec[i] << " \n"[i + 1 == vec_len];
    }
    return os;
}

// Main
struct Edge {
    int to;
    ll cost;
    Edge(int t, ll c) : to(t), cost(c) {}
};
constexpr int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

int main() {
    int n, m;
    cin >> n >> m;
    vector<int> h(m), w(m), c(m);
    rep(i, m) {
        cin >> h[i] >> w[i] >> c[i];
        --h[i];
        --w[i];
    }
    
    vector<vector<int>> vec(n, vector<int>(n, 0));
    rep(i, m) vec[h[i]][w[i]] = c[i];
    
    vector<vector<Edge>> g(n * n * 2);
    rep(i, n) rep(j, n) rep(k, 4) {
        if (i + dx[k] < 0 || i + dx[k] >= n || j + dy[k] < 0 || j + dy[k] >= n) continue;
        if (!vec[i + dx[k]][j + dy[k]]) {
            g[(i * n + j) * 2 + 0].emplace_back(((i + dx[k]) * n + j + dy[k]) * 2 + 0, 1);
            g[(i * n + j) * 2 + 1].emplace_back(((i + dx[k]) * n + j + dy[k]) * 2 + 1, 1);
        } else {
            g[(i * n + j) * 2 + 0].emplace_back(((i + dx[k]) * n + j + dy[k]) * 2 + 1, 1);
            g[(i * n + j) * 2 + 0].emplace_back(((i + dx[k]) * n + j + dy[k]) * 2 + 0, vec[i + dx[k]][j + dy[k]] + 1);
            g[(i * n + j) * 2 + 1].emplace_back(((i + dx[k]) * n + j + dy[k]) * 2 + 1, vec[i + dx[k]][j + dy[k]] + 1);
        }
    }
    
    vector<ll> dist(n * n * 2, INF);
    priority_queue<pair<ll, int>> pq;
    dist[0] = 0;
    pq.emplace(dist[0], 0);
    while (!pq.empty()) {
        auto [d, v] = pq.top();
        pq.pop();
        d *= -1;
        if (dist[v] != d) continue;
        for (Edge &e : g[v]) {
            if (chmin(dist[e.to], d + e.cost)) pq.emplace(-dist[e.to], e.to);
        }
    }
    
    cout << min(dist[(n * n - 1) * 2], dist[(n * n - 1) * 2 + 1]) << "\n";
}