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

    N,M=map(int,input().split())
    inf=float("inf")

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

    V=[(1,0),(-1,0),(0,1),(0,-1)]

    Dist=[[[inf,inf] for _ in range(N+1)] for __ in range(N+1)]
    Dist[1][1][0]=0

    Q=[(0,(1,1,0))]
    heapq.heapify(Q)

    while Q:
        d,v=heapq.heappop(Q)

        i,j,m=v
        if d<Dist[i][j][m]:
            continue

        if i==j==N:
            break

        for (a,b) in V:
            if 1<=i+a<=N and 1<=j+b<=N:
                if d+F[i+a][j+b]<Dist[i+a][j+b][m]:
                    Dist[i+a][j+b][m]=d+F[i+a][j+b]
                    heapq.heappush(Q,(d+F[i+a][j+b],(i+a,j+b,m)))

                if m==0 and d+1<Dist[i+a][j+b][1]:
                    Dist[i+a][j+b][1]=d+1
                    heapq.heappush(Q,(d+1,(i+a,j+b,1)))

    print(min(Dist[N][N]))

if __name__=="__main__":
    main()