import heapq
N,M = map(int,input().split())
C = [[0 for _ in range(N+1)] for _ in range(N+1)]
A = []
for _ in range(M):
    h,w,c = map(int,input().split())
    C[h][w] = c
    A.append((h,w))
INFTY = 10**12+10000
dist = [[[INFTY for _ in range(2)] for _ in range(N+1)] for _ in range(N+1)]
dist[1][1][0] = 0
visited = [[[False for _ in range(2)] for _ in range(N+1)] for _ in range(N+1)]
heap = [(0,1,1,0)]
while heap:
    d,i,j,s = heapq.heappop(heap)
    if dist[i][j][s]<d:continue
    visited[i][j][s] = True
    for di,dj in [[0,1],[-1,0],[0,-1],[1,0]]:
        if s==1:
            if 1<=i+di<=N and 1<=j+dj<=N and visited[i+di][j+dj][1]==False and dist[i+di][j+dj][1]>d+1+C[i+di][j+dj]:
                dist[i+di][j+dj][1] = d+1+C[i+di][j+dj]
                heapq.heappush(heap,(d+1+C[i+di][j+dj],i+di,j+dj,1))
        else:
            if 1<=i+di<=N and 1<=j+dj<=N and visited[i+di][j+dj][0]==False and dist[i+di][j+dj][0]>d+1+C[i+di][j+dj]:
                dist[i+di][j+dj][0] = d+1+C[i+di][j+dj]
                heapq.heappush(heap,(d+1+C[i+di][j+dj],i+di,j+dj,0))
            if 1<=i+di<=N and 1<=j+dj<=N and visited[i+di][j+dj][1]==False and dist[i+di][j+dj][1]>d+1:
                dist[i+di][j+dj][1] = d+1
                heapq.heappush(heap,(d+1,i+di,j+dj,1))
print(min(dist[N][N]))