#region Header
#!/usr/bin/env python3
# from typing import *

import sys
import io
import math
import collections
import decimal
import itertools
from queue import PriorityQueue
import bisect
import heapq

def input():
    return sys.stdin.readline()[:-1]

sys.setrecursionlimit(1000000)
#endregion

# _INPUT = """10 34
# 7 5 2286
# 2 2 3140
# 9 3 4600
# 9 6 5277
# 9 4 9057
# 1 6 1156
# 1 7 2356
# 6 10 2106
# 9 5 1192
# 4 6 9022
# 8 2 3221
# 8 9 7792
# 4 3 609
# 4 7 6593
# 2 6 3097
# 8 6 4348
# 2 10 213
# 7 3 7105
# 10 9 4412
# 6 4 4410
# 8 5 2870
# 2 5 2185
# 2 7 8469
# 7 10 3727
# 5 5 4076
# 10 6 2106
# 1 4 385
# 1 8 9753
# 6 7 3240
# 9 2 4081
# 3 7 4784
# 6 1 6748
# 5 10 6062
# 2 3 7218
# """
# sys.stdin = io.StringIO(_INPUT)


def f(N, P):
    dist = [[10**10 for _ in range(N)] for _ in range(N)]
    hq = []
    heapq.heappush(hq, (0, (0, 0)))
    dist[0][0] = 0
    path = [[None for _ in range(N)] for _ in range(N)]
    while hq:
        d, (h, w) = heapq.heappop(hq)
        if d > dist[h][w]:
            continue
        for (dh, dw) in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
            (h1, w1) = (h + dh, w + dw)
            if 0 <= h1 <= N-1 and 0 <= w1 <= N-1:
                d1 = dist[h][w] + P[h1][w1]
                if d1 < dist[h1][w1]:
                    dist[h1][w1] = d1
                    path[h1][w1] = (h, w)
                    heapq.heappush(hq, (d1, (h1, w1)))

    return dist[N-1][N-1]


def main():
    N, M = map(int, input().split())
    P = [[1 for _ in range(N)] for _ in range(N)]
    P[0][0] = 0

    points = []
    for _ in range(M):
        h, w, c = map(int, input().split())
        h -= 1
        w -= 1
        points.append((h, w, c))
        P[h][w] = c + 1

    n_min = 10**10
    for i in range(M):
        h, w, c = points[i]
        P[h][w] = 1
        n = f(N, P)
        n_min = min(n_min, n)
        P[h][w] = c + 1

    print(n_min)


if __name__ == '__main__':
    main()