import heapq


class Graph():
    def __init__(self, N, Type):
        self._V = N  # 頂点の数
        self._E = 0  # 辺の数
        self.type = Type
        if Type == "D":
            # 2次元リスト
            self.G = [[] for i in range(self._V+1)]
        elif Type == "W":
            # 隣接行列
            self.G = [[float("inf") for i in range(self._V+1)]
                      for j in range(self._V+1)]
        elif Type == "B" or Type == "K":
            # 1次元リスト
            self.G = []

    def E(self):
        """ 辺数 """
        return self._E

    @property
    def V(self):
        """ 頂点数 """
        return self._V

    def add(self, _from, _to, _cost):
        """ 2頂点と、辺のコストを追加する """
        self._E += 1
        if self.type == "D":
            self.G[_from].append((_to, _cost))

        elif self.type == "W":
            self.G[_from][_to] = _cost

        elif self.type == "B" or self.type == "K":
            self.G.append((_from, _to, _cost))

    def add_both(self, _from, _to, _cost):
        """ 2頂点と、辺のコストを追加する (無向グラフ) """
        self.add(_from, _to, _cost)
        self.add(_to, _from, _cost)


class Dijkstra(Graph):
    def __init__(self, N):
        super().__init__(N, "D")
        self.d = [10**20]*(self._V + 1)
        self.que = []
        heapq.heapify(self.que)

    def shortest_path(self, s):
        for i in range(self._V+1):
            self.d[i] = 10**20
        self.d[s] = 0
        heapq.heappush(self.que, (0, s))
        while self.que:
            cost, v = heapq.heappop(self.que)
            if self.d[v] < cost:
                continue

            for node, cost in self.G[v]:
                if self.d[node] > self.d[v] + cost:
                    self.d[node] = self.d[v] + cost
                    heapq.heappush(self.que, (self.d[node], node))
        return self.d


N, M = map(int, input().split())
dif = [(-1, 0), (1, 0), (0, 1), (0, -1)]
cost = [0]*(N**2)
G = Dijkstra(N**2)
Gs = Dijkstra(N**2)

def press(i, j):
    return N*i + j


for i in range(M):
    h, w, c = map(int, input().split())
    h -= 1
    w -= 1
    cost[press(h, w)] = c

inf = 1 << 60


for i in range(N):
    for j in range(N):
        now = press(i, j)
        for dx, dy in dif:
            if 0 <= dx + i < N and 0 <= dy + j < N:
                adj = press(dx+i, dy + j)
                G.add(now, adj, cost[adj] + 1)
                Gs.add(now,adj, cost[now] + 1)

G_s = G.shortest_path(0)
Gs_g = Gs.shortest_path(N**2 - 1)
ans = inf
for i in range(1, N**2 - 1):
    ans = min(ans, G_s[i] + Gs_g[i] - cost[i])
print(ans)