#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 7
# 1 2 5
# 1 3 8
# 2 1 7
# 2 2 3
# 2 3 9
# 3 1 11
# 3 2 2
# """
# 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

    # dp[0] : 無料の権利を使っていない
    # dp[1] : 無料の権利を使った
    dp = [[[10**10 for _ in range(N)] for _ in range(N)] for _ in range(2)]

    hq = []
    heapq.heappush(hq, (0, (0, 0, 0)))
    dp[0][0][0] = 0
    while hq:
        d, (flag, h, w) = heapq.heappop(hq)
        if d > dp[flag][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:
                if flag == 0:
                    d1 = dp[0][h][w] + P[h1][w1]
                    if d1 < dp[0][h1][w1]:
                        dp[0][h1][w1] = d1
                        heapq.heappush(hq, (d1, (0, h1, w1)))
                    d2 = dp[0][h][w] + 1
                    if d2 < dp[1][h1][w1]:
                        dp[1][h1][w1] = d2
                        heapq.heappush(hq, (d2, (1, h1, w1)))
                else:
                    d1 = dp[1][h][w] + P[h1][w1]
                    if d1 < dp[1][h1][w1]:
                        dp[1][h1][w1] = d1
                        heapq.heappush(hq, (d1, (1, h1, w1)))

    print(min(dp[0][N-1][N-1], dp[1][N-1][N-1]))


if __name__ == '__main__':
    main()