import sys
readline=sys.stdin.readline
import heapq

N,M=map(int,readline().split())
C=[[0]*N for h in range(N)]
for m in range(M):
    h,w,c=map(int,readline().split())
    h-=1;w-=1
    C[h][w]=c
inf=1<<60
dist=[[[inf]*N for h in range(N)] for b in range(2)]
dist[0][0][0]=0
queue=[(0,0,0,0)]
while queue:
    dx,bx,hx,wx=heapq.heappop(queue)
    if dist[bx][hx][wx]<dx:
        continue
    for dh,dw in ((0,1),(1,0),(0,-1),(-1,0)):
        hy=hx+dh
        wy=wx+dw
        if 0<=hy<N and 0<=wy<N:
            if dist[bx][hy][wy]>dx+C[hy][wy]+1:
                dist[bx][hy][wy]=dx+C[hy][wy]+1
                heapq.heappush(queue,(dist[bx][hy][wy],bx,hy,wy))
            if bx==0:
                if dist[1][hy][wy]>dx+1:
                    dist[1][hy][wy]=dx+1
                    heapq.heappush(queue,(dist[1][hy][wy],1,hy,wy))
ans=dist[1][N-1][N-1]
print(ans)