import heapq
n,m=map(int,input().split())
money=[[0]*n for _ in range(n)]
for _ in range(m):
    h,w,c=map(int,input().split())
    money[h-1][w-1]=c
inf=float('inf')
d=[[[inf]*2 for _ in range(n)] for _ in range(n)]
d[0][0][0]=0
q=[(0,0,0,0)]
while q:
    now_d,y,x,use=heapq.heappop(q)
    if d[y][x][use]<now_d:
        continue
    for dy,dx in [(1,0),(0,1),(-1,0),(0,-1)]:
        ny=y+dy;nx=x+dx
        if 0<=ny<n and 0<=nx<n:
            if money[ny][nx]==0:
                if d[ny][nx][use]>now_d+1:
                    d[ny][nx][use]=now_d+1
                    heapq.heappush(q,(now_d+1,ny,nx,use))
            else:
                cost=money[ny][nx]
                if d[ny][nx][use]>now_d+cost+1:
                    d[ny][nx][use]=now_d+cost+1
                    heapq.heappush(q,(now_d+cost+1,ny,nx,use))
                if use==0:
                    if d[ny][nx][1]>now_d+1:
                        d[ny][nx][1]=now_d+1
                        heapq.heappush(q,(now_d+1,ny,nx,1))
print(d[-1][-1][1])