mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.buffer.readline
    import heapq

    def dijkstra(adj, start):
        # adj: [[to, cost] * vertices], 0th index must be empty
        inf = 1 << 60
        dist = [inf] * len(adj)
        dist[start] = 0
        q = []
        heapq.heappush(q, (0, start))
        while q:
            min_dist, v_from = heapq.heappop(q)
            if min_dist > dist[v_from]:
                continue
            v_tos = adj[v_from]
            for v_to, cost in v_tos:
                if min_dist + cost < dist[v_to]:
                    dist[v_to] = min_dist + cost
                    heapq.heappush(q, (dist[v_to], v_to))
        return dist

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

    d = [(0, 1), (0, -1), (1, 0), (-1, 0)]
    adj = [[] for _ in range(N*N + 1)]
    for h in range(1, N+1):
        for w in range(1, N+1):
            hw = (h-1) * N + w
            for dh, dw in d:
                h_new, w_new = h+dh, w+dw
                if 1 <= h_new <= N and 1 <= w_new <= N:
                    hw_new = (h_new - 1) * N + w_new
                    adj[hw].append((hw_new, grid[h_new][w_new] + 1))

    dist1 = dijkstra(adj, 1)
    dist2 = dijkstra(adj, N*N)
    ans = float("inf")
    for h in range(1, N+1):
        for w in range(1, N+1):
            hw = (h-1) * N + w
            ans = min(ans, dist1[hw] + dist2[hw] - grid[h][w] * 2)
    print(ans)


if __name__ == '__main__':
    main()