import sys
input = sys.stdin.readline

N,M=map(int,input().split())

DP=[[1<<60]*N for i in range(N)]
DP2=[[1<<60]*N for i in range(N)]
MAP=[[1]*N for i in range(N)]

for i in range(M):
    h,w,c=map(int,input().split())
    h-=1
    w-=1
    MAP[h][w]+=c

DP[0][0]=0

import heapq

# Q=[(c,x,y,f)]
Q=[(0,0,0,0)]

while Q:
    c,x,y,f=heapq.heappop(Q)

    if f==0 and c>DP[x][y]:
        continue
    if f==1 and c>DP2[x][y]:
        continue

    if f==1:
        for z,w in [(x+1,y),(x-1,y),(x,y+1),(x,y-1)]:
            if 0<=z<N and 0<=w<N and c+MAP[z][w]<DP2[z][w]:
                DP2[z][w]=c+MAP[z][w]
                heapq.heappush(Q,(DP2[z][w],z,w,1))

    else:
        for z,w in [(x+1,y),(x-1,y),(x,y+1),(x,y-1)]:
            if 0<=z<N and 0<=w<N and c+MAP[z][w]<DP[z][w]:
                DP[z][w]=c+MAP[z][w]
                heapq.heappush(Q,(DP[z][w],z,w,0))

            if 0<=z<N and 0<=w<N and MAP[z][w]!=1 and c+1<DP2[z][w]:
                DP2[z][w]=c+1
                heapq.heappush(Q,(DP2[z][w],z,w,1))
        
print(min(DP[N-1][N-1],DP2[N-1][N-1]))