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]))