import sys import collections,heapq sys.setrecursionlimit(10**7) def I(): return int(sys.stdin.readline().rstrip()) def MI(): return map(int,sys.stdin.readline().rstrip().split()) def LI(): return list(map(int,sys.stdin.readline().rstrip().split())) def LI2(): return list(map(int,sys.stdin.readline().rstrip())) def S(): return sys.stdin.readline().rstrip() def LS(): return list(sys.stdin.readline().rstrip().split()) def LS2(): return list(sys.stdin.readline().rstrip()) class Dijkstra: def __init__(self): self.e = collections.defaultdict(list) def add(self, u, v, d, directed=False): if directed is False: # 無向グラフの場合 self.e[u].append([v, d]) self.e[v].append([u, d]) else: # 有向グラフの場合 self.e[u].append([v, d]) def delete(self, u, v): self.e[u] = [_ for _ in self.e[u] if _[0] != v] def search(self, s): # sから各頂点までの最短距離 d = collections.defaultdict(lambda: float('inf')) d[s] = 0 q = [] heapq.heappush(q, (0, s)) v = collections.defaultdict(bool) while len(q): k, u = heapq.heappop(q) if v[u]: continue v[u] = True for uv, ud in self.e[u]: if v[uv]: continue vd = k + ud if d[uv] > vd: d[uv] = vd heapq.heappush(q, (vd, uv)) return d N,M = MI() HWC = [tuple(MI()) for _ in range(M)] Di = Dijkstra() for i in range(1,N+1): for j in range(1,N): Di.add((i,j),(i,j+1),1,directed=False) for i in range(1,N): for j in range(1,N+1): Di.add((i,j),(i+1,j),1,directed=False) for h,w,c in HWC: if h >= 2: Di.delete((h-1,w),(h,w)) Di.add((h-1,w),(h,w),c+1,directed=True) if h <= N-1: Di.delete((h+1,w),(h,w)) Di.add((h+1,w),(h,w),c+1,directed=True) if w >= 2: Di.delete((h,w-1),(h,w)) Di.add((h,w-1),(h,w),c+1,directed=True) if w <= N-1: Di.delete((h,w+1),(h,w)) Di.add((h,w+1),(h,w),c+1,directed=True) dist = Di.search((1,1)) dist2 = Di.search((N,N)) ans = dist[(N,N)] for h,w,c in HWC: ans = min(ans,dist[(h,w)]+dist2[(h,w)]-2*c) print(ans)