import heapq

debug=False
def pprint(*s):
    if debug==True:
        print(*s)

n,m=map(int,input().split())
fees={}
for _ in range(m):
    y,x,cost=map(int,input().split())
    fees[(x-1,y-1)]=cost
INF=10**18
costs=[[[INF,INF] for _ in range(n)] for _ in range(n)]
costs[0][0]=[0,0]
q=[]
heapq.heappush(q,(0,0,0,0))
while len(q)!=0:
    curcost,x,y,skipped=heapq.heappop(q)
    pprint('x:{},y:{},cost:{},skipped:{}'.format(x,y,curcost,skipped))
    if x==n-1 and y==n-1:
        print(curcost)
        exit()
    for dx,dy in [(-1,0),(1,0),(0,-1),(0,1)]:
        if 0<=x+dx<=n-1 and 0<=y+dy<=n-1:
            if (x+dx,y+dy) in fees:
                if skipped==0:
                    if costs[y+dy][x+dx][skipped+1]>curcost+1:
                        heapq.heappush(q,(curcost+1,x+dx,y+dy,skipped+1))
                        costs[y+dy][x+dx][skipped+1]=curcost+1
                    if costs[y+dy][x+dx][skipped]>curcost+1+fees[(x+dx,y+dy)]:
                        heapq.heappush(q,(curcost+1+fees[(x+dx,y+dy)],x+dx,y+dy,skipped))
                        costs[y+dy][x+dx][skipped]=curcost+1+fees[(x+dx,y+dy)]
                else:
                    if costs[y+dy][x+dx][skipped]>curcost+1+fees[(x+dx,y+dy)]:
                        heapq.heappush(q,(curcost+1+fees[(x+dx,y+dy)],x+dx,y+dy,skipped))
                        costs[y+dy][x+dx][skipped]=curcost+1+fees[(x+dx,y+dy)]
            else:
                if costs[y+dy][x+dx][skipped]>curcost+1:
                    heapq.heappush(q,(curcost+1,x+dx,y+dy,skipped))
                    costs[y+dy][x+dx][skipped]=curcost+1