import sys
import io, os
input = io.BytesIO(os.read(0,os.fstat(0).st_size)).readline

n, m = map(int, input().split())
HWC = []
A = [[0]*n for _ in range(n)]
for i in range(m):
    h, w, c = map(int, input().split())
    h, w =h-1, w-1
    HWC.append((h, w, c))
    A[h][w] = c

import heapq
INF = 10**18
def dijkstra_heap(s, edge):
    n = len(edge)
    d = [INF] * n
    used = [True] * n #True: not used
    d[s] = 0
    used[s] = False
    edgelist = []
    for e in edge[s]:
        heapq.heappush(edgelist,e)
    while len(edgelist):
        minedge = heapq.heappop(edgelist)
        if not used[minedge[1]]:
            continue
        v = minedge[1]
        d[v] = minedge[0]
        used[v] = False
        for e in edge[v]:
            if used[e[1]]:
                heapq.heappush(edgelist,(e[0]+d[v],e[1]))
    return d

ans = INF

g = [[] for _ in range(n**2)]
rg = [[] for _ in range(n**2)]
for i in range(n):
    for j in range(n):
        v = i*n+j
        for di, dj in (1, 0), (0, 1):
            ni, nj = i+di, j+dj
            if 0 <= ni < n and 0 <= nj < n:
                c = A[ni][nj]+1
                u = ni*n+nj
                g[v].append((c, u))
                rg[u].append((c, v))
d = dijkstra_heap(0, g)
#print(d)
cur = n**2-1
X = []
while cur != 0:
    for c, u in rg[cur]:
        if d[u]+c == d[cur]:
            cur = u
            X.append(c-1)
#print(X)
ans = d[-1]-max(X)
print(ans)