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
dist0=[[inf]*N for h in range(N)]
dist1=[[inf]*N for h in range(N)]
dist0[0][0]=0
queue=[(0,0,0,0)]
while queue:
    dx,bx,hx,wx=heapq.heappop(queue)
    if bx==0:
        if dist0[hx][wx]<dx:
            continue
    else:
        if dist1[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 bx==0:
                if dist0[hy][wy]>dx+C[hy][wy]+1:
                    dist0[hy][wy]=dx+C[hy][wy]+1
                    heapq.heappush(queue,(dist0[hy][wy],bx,hy,wy))
                if dist1[hy][wy]>dx+1:
                    dist1[hy][wy]=dx+1
                    heapq.heappush(queue,(dist1[hy][wy],1,hy,wy))
            else:
                if dist1[hy][wy]>dx+C[hy][wy]+1:
                    dist1[hy][wy]=dx+C[hy][wy]+1
                    heapq.heappush(queue,(dist1[hy][wy],bx,hy,wy))
ans=dist1[N-1][N-1]
print(ans)