from collections import deque

INF = float('inf')

N, M = map(int, input().split())

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

dp = [[(INF, 0)] * N for _ in range(N)]
dp[0][0] = (0, 0)
q = deque([(0, 0)])
while q:
    h, w = q.popleft()
    t, m = dp[h][w]
    if h != 0:
        a, b = dp[h - 1][w]
        d = t + 1 + c[h - 1][w]
        e = max(m, c[h - 1][w])
        if d - e < a - b:
            dp[h - 1][w] = (d, e)
            q.append((h - 1, w))
    if h != N - 1:
        a, b = dp[h + 1][w]
        d = t + 1 + c[h + 1][w]
        e = max(m, c[h + 1][w])
        if d - e < a - b:
            dp[h + 1][w] = (d, e)
            q.append((h + 1, w))
    if w != 0:
        a, b = dp[h][w - 1]
        d = t + 1 + c[h][w - 1]
        e = max(m, c[h][w - 1])
        if d - e < a - b:
            dp[h][w - 1] = (d, e)
            q.append((h, w - 1))
    if w != N - 1:
        a, b = dp[h][w + 1]
        d = t + 1 + c[h][w + 1]
        e = max(m, c[h][w + 1])
        if d - e < a - b:
            dp[h][w + 1] = (d, e)
            q.append((h, w + 1))
t, m = dp[N - 1][N - 1]
print(t - m)