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)]

    Dist0=[[inf]*(N+1) for __ in range(N+1)]
    Dist1=[[inf]*(N+1) for __ in range(N+1)]
    Dist0[1][1]=0

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

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

        i,j,m=v
        if (m==0 and d<Dist0[i][j]) or (m==1 and d<Dist1[i][j]):
            continue

        if i==j==N:
            break

        for (a,b) in V:
            if 1<=i+a<=N and 1<=j+b<=N:
                c=d+F[i+a][j+b]
                if m==0 and c<Dist0[i+a][j+b]:
                    Dist0[i+a][j+b]=c
                    heapq.heappush(Q,(c,(i+a,j+b,0)))

                if m==1 and c<Dist1[i+a][j+b]:
                    Dist1[i+a][j+b]=c
                    heapq.heappush(Q,(c,(i+a,j+b,1)))

                if m==0 and d+1<Dist1[i+a][j+b]:
                    Dist1[i+a][j+b]=d+1
                    heapq.heappush(Q,(d+1,(i+a,j+b,1)))

    print(min(Dist0[N][N],Dist1[N][N]))

if __name__=="__main__":
    main()