#include "bits/stdc++.h"
using namespace std;
//#include "atcoder/all"
//using namespace atcoder;
#define int long long
#define REP(i, n) for (int i = 0; i < (int)n; ++i)
#define RREP(i, n) for (int i = (int)n - 1; i >= 0; --i)
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define RFOR(i, s, n) for (int i = (int)n - 1; i >= s; --i)
#define ALL(a) a.begin(), a.end()
#define IN(a, x, b) (a <= x && x < b)
template<class T>istream&operator >>(istream&is,vector<T>&vec){for(T&x:vec)is>>x;return is;}
template<class T>inline void out(T t){cout << t << "\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout << t << " ";out(ts...);}
template<class T>inline bool CHMIN(T&a,T b){if(a > b){a = b;return true;}return false;}
template<class T>inline bool CHMAX(T&a,T b){if(a < b){a = b;return true;}return false;}
constexpr int INF = 1e18;

int dist[505][505][2];
vector<pair<int,int>>dir = {{1,0},{0,1},{-1,0},{0,-1}};
const int dx[] = {0, 1, 1, 1, 0, -1, -1, -1, 0};
const int dy[] = {1, 1, 0, -1, -1, -1, 0, 1, 1};

signed main(){
	int N, M;
	cin >> N >> M;
	vector<vector<int>>fee(N, vector<int>(N));
	REP(i, M) {
		int h, w, c;
		cin >> h >> w >> c;
		fee[h - 1][w - 1] = c;
	}
	REP(i, N) REP(j, N) fee[i][j] += 1;
	REP(i, 505) REP(j, 505) REP(k, 2) dist[i][j][k] = INF;
	dist[0][0][0] = 0;
	using T = tuple<int, int, int, int>;
	priority_queue<T, vector<T>, greater<T>> que;
	que.push({0, 0, 0, 0});
	while(que.size()) {
		auto[cost, i, j, free] = que.top();
		que.pop();
		if(dist[i][j][free] != cost) continue;
		for(auto[dx, dy]: dir) {
			int ni = i + dx;
			int nj = j + dy;
			if(!IN(0, ni, N) || !IN(0, nj, N)) continue;
			if(CHMIN(dist[ni][nj][free], cost + fee[ni][nj])) {
				que.push({dist[ni][nj][free], ni, nj, free});
			}
			if(free == 0 && CHMIN(dist[ni][nj][1], cost + 1)) {
				que.push({dist[ni][nj][1], ni, nj, 1});
			}
		}
	}
	out(min(dist[N - 1][N - 1][0], dist[N - 1][N - 1][1]));
}