import sys
readline = sys.stdin.readline
from heapq import heappush, heappop
inf = float('inf')


def dijkstra(s, g, N):
    # ゴールがない場合はg=-1とする。
    g0 = f(N-1, N-1, 0)
    g1 = f(N-1, N-1, 1)

    def cost(v, m):
        return v * N + m

    dist = [inf] * N
    mindist = [inf] * N
    seen = [False] * N
    Q = [cost(0, s)]
    while Q:
        c, m = divmod(heappop(Q), N)
        if seen[m]:
            continue
        seen[m] = True
        dist[m] = c
        if m == g0 or m == g1:
            return dist

        #------heapをアップデートする。--------
        for u, C in G[m]:
            if seen[u]:
                continue
            newdist = dist[m] + C

            #------------------------------------
            if newdist >= mindist[u]:
                continue
            mindist[u] = newdist
            heappush(Q, cost(newdist, u))
    return dist

def f(h, w, c):
    return h + N * w + N * N * c

N, M = map(int, readline().split())
temp = [[0] * N for i in range(N)]
for i in range(M):
    h, w, c = map(int, readline().split())
    h, w = h - 1, w - 1
    temp[h][w] = c
    
G = [[] for i in range(N * N * 2)]
dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]
for i in range(N):
    for j in range(N):
        for k in range(4):
            x = i + dx[k]
            y = j + dy[k]
            if x < 0 or x > N - 1 or y < 0 or y > N - 1:
                continue
            G[f(i, j, 0)].append( ( f(x, y, 0), temp[x][y] + 1))
            G[f(x, y, 0)].append( ( f(i, j, 0), temp[i][j] + 1))
            G[f(i, j, 1)].append( ( f(x, y, 1), temp[x][y] + 1))
            G[f(x, y, 1)].append( ( f(i, j, 1), temp[i][j] + 1))
            G[f(i, j, 0)].append( ( f(x, y, 1), 1))
            G[f(x, y, 0)].append( ( f(i, j, 1), 1))
            
D = dijkstra(0, f(N - 1, N - 1, 1), N * N * 2)
print(min(D[f(N - 1, N - 1, 0)], D[f(N - 1, N - 1, 1)]))