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), (1, 0), (0, 1), (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)