#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N, M;
	cin >> N >> M;
	vector C(N, vector<int>(N));
	REP(i, M) {
		int h, w, c;
		scanf("%d %d %d", &h, &w, &c);
		C[h - 1][w - 1] = c;
	}
	vector dp(N, vector(N, vector(2, longINF)));
	using tup = tuple<ll, int, int>;
	priority_queue<tup, vector<tup>, greater<tup>> pq;
	dp[0][0][0] = 0;
	pq.emplace(0, 0, 0);
	while (!pq.empty()) {
		auto [c, p, f] = pq.top(); pq.pop();
		int y = p / N, x = p % N;
		if (c != dp[y][x][f]) continue;
		int dx[] {1,0,-1,0}, dy[] {0,1,0,-1};
		REP(dir, 4) {
			const int ny = y + dy[dir], nx = x + dx[dir];
			if (ny < 0 || ny >= N || nx < 0 || nx >= N) continue;
			if (chmin(dp[ny][nx][f], dp[y][x][f] + C[ny][nx] + 1)) pq.emplace(dp[ny][nx][f], ny * N + nx, f);
			if (!f && chmin(dp[ny][nx][1], dp[y][x][f] + 1)) pq.emplace(dp[ny][nx][1], ny * N + nx, 1);
		}
	}
	
	cout << dp[N - 1][N - 1][1] << endl;
}