def main(): import sys,heapq input=sys.stdin.readline N,M=map(int,input().split()) inf=float("inf") F=[[1]*(N+1) for _ in range(N+1)] for _ in range(M): h,w,c=map(int,input().split()) F[h][w]+=c V=[(1,0),(-1,0),(0,1),(0,-1)] Dist=[[[inf,inf] for _ in range(N+1)] for __ in range(N+1)] Dist[1][1][0]=0 Q=[(0,(1,1,0))] heapq.heapify(Q) while Q: d,v=heapq.heappop(Q) i,j,m=v if d<Dist[i][j][m]: continue if i==j==N: break for (a,b) in V: if 1<=i+a<=N and 1<=j+b<=N: if d+F[i+a][j+b]<Dist[i+a][j+b][m]: Dist[i+a][j+b][m]=d+F[i+a][j+b] heapq.heappush(Q,(d+F[i+a][j+b],(i+a,j+b,m))) if m==0 and d+1<Dist[i+a][j+b][1]: Dist[i+a][j+b][1]=d+1 heapq.heappush(Q,(d+1,(i+a,j+b,1))) print(min(Dist[N][N])) if __name__=="__main__": main()