import heapq

INF = 1 << 60

n, m = map(int, input().split())
cost = [[0 for _ in range(n)] for _ in range(n)]
for _ in range(m):
  hi, wi, ci = map(int, input().split())
  cost[hi - 1][wi - 1] = ci

dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]

dist = [[[INF for _ in range(n)] for _ in range(n)] for _ in range(2)]
dist[0][0][0] = 0
hp = [(0, 0, 0, 0)]
while len(hp) > 0:
  cd, ci, cx, cy = heapq.heappop(hp)
  if cd > dist[ci][cx][cy]:
    continue

  for d in range(4):
    nx = cx + dx[d]
    if nx < 0 or nx >= n:
      continue
    ny = cy + dy[d]
    if ny < 0 or ny >= n:
      continue

    if dist[ci][nx][ny] > dist[ci][cx][cy] + cost[nx][ny] + 1:
      dist[ci][nx][ny] = dist[ci][cx][cy] + cost[nx][ny] + 1
      heapq.heappush(hp, (dist[ci][nx][ny], ci, nx, ny))

    if ci == 0 and dist[1][nx][ny] > dist[ci][cx][cy] + 1:
      dist[1][nx][ny] = dist[ci][cx][cy] + 1
      heapq.heappush(hp, (dist[1][nx][ny], 1, nx, ny))

print(min(dist[0][-1][-1], dist[1][-1][-1]))