import heapq


dd = ((1, 0), (-1, 0), (0, 1), (0, -1))
def dijkstra() -> list:
    INF = 10 ** 18
    dist = [[[INF] * n for i in range(n)] for i in range(2)]
    dist[0][0][0] = 0
    q = [(0, 0, 0, 0)] # q = [(startからの距離, i, j, cnt)]
    while q:
        d, i, j, cnt = heapq.heappop(q)
        if dist[cnt][i][j] < d:
            continue
        for di, dj in dd:
            nxt_i, nxt_j = i + di, j + dj
            if not(0 <= nxt_i < h and 0 <= nxt_j < w):
                continue
            if cnt == 1:
                if dist[1][i][j] + 1 + cost[nxt_i][nxt_j] < dist[1][nxt_i][nxt_j]:
                    dist[1][nxt_i][nxt_j] = dist[1][i][j] + 1 + cost[nxt_i][nxt_j]
                    heapq.heappush(q, (dist[1][nxt_i][nxt_j], nxt_i, nxt_j, 1))
            else:
                if dist[0][i][j] + 1 + cost[nxt_i][nxt_j] < dist[0][nxt_i][nxt_j]:
                    dist[0][nxt_i][nxt_j] = dist[0][i][j] + 1 + cost[nxt_i][nxt_j]
                    heapq.heappush(q, (dist[0][nxt_i][nxt_j], nxt_i, nxt_j, 0))
                if dist[0][i][j] + 1 < dist[1][nxt_i][nxt_j]:
                    dist[1][nxt_i][nxt_j] = dist[0][i][j] + 1
                    heapq.heappush(q, (dist[1][nxt_i][nxt_j], nxt_i, nxt_j, 1))
    return dist


n, m = map(int, input().split())
h, w = n, n
info = [list(map(int, input().split())) for i in range(m)]


cost = [[0] * n for i in range(n)]
for i, j, c in info:
    i -= 1
    j -= 1
    cost[i][j] = c

dist = dijkstra()
print(dist[1][-1][-1])