#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 = """3 2
# 2 2 100
# 2 3 200
# """
# sys.stdin = io.StringIO(_INPUT)



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

    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)))

    M = 0
    (h, w) = (N-1, N-1)
    while (h, w) != (0, 0):
        M = max(M, P[h][w])
        (h, w) = path[h][w]

    print(dist[N-1][N-1] - (M - 1))

if __name__ == '__main__':
    main()