#O(ElogV),重みが整数の場合のみ
import heapq
def dijkstra_fast(s,N,edge,mod):
  dists = [float('inf')] * N #始点sから各頂点への最短距離
  used = [False] * N
  dists[s] = 0
  used[s] = True
  vlist = []
  #vlist : [sからの暫定(未確定)最短距離,頂点]のリスト
  #edge[s] : sから出る枝の[重み,終点]のリスト
  for w,v in edge[s]:
    heapq.heappush(vlist,w*mod+v) #sの隣の点は枝の重さがそのまま暫定最短距離となる
  while len(vlist):
    #まだ使われてない頂点の中から最小の距離のものを探す→確定させる
    minedge = heapq.heappop(vlist)
    #minedge : sからの(確定)最短距離*mod+頂点
    d,v = divmod(minedge,mod)
    if used[v]:
      continue
    dists[v] = d
    used[v] = True
    for d,w in edge[v]:
      if not used[w]:
        heapq.heappush(vlist,(dists[v]+d)*mod+w)
  return dists

N, M = map(int, input().split())
from collections import defaultdict
d = defaultdict(lambda: 1)
for i in range(M):
  h,w,c = map(int, input().split())
  d[(h-1)*N+w-1] = 1+c
edge = [[] for _ in range(N*N)]
for h in range(N):
  for w in range(N):
    if h>0:
      to = (h-1)*N+w
      edge[h*N+w].append((d[to], to))
    if w>0:
      to = h*N+w-1
      edge[h*N+w].append((d[to], to))
    if h<N-1:
      to = (h+1)*N+w
      edge[h*N+w].append((d[to], to))
    if w<N-1:
      to = h*N+w+1
      edge[h*N+w].append((d[to], to))
dist1 = dijkstra_fast(0, N*N, edge, 10**6)
dist2 = dijkstra_fast(N*N-1, N*N, edge, 10**6)
ans = float('inf')
for i in range(1,N*N-1):
  ans = min(ans, dist1[i]+dist2[i]-(d[i]-1)*2)
print(ans)