import heapq
import sys
input=sys.stdin.readline

n,m=map(int,input().split())
fees=[1]*(n*n)
for _ in range(m):
    y,x,cost=map(int,input().split())
    fees[n*(y-1)+x-1]+=cost
g=[[] for _ in range(2*n*n)]
for y in range(n):
    for x in range(n):
        for dx,dy in [(0,1),(0,-1),(1,0),(-1,0)]:
            if 0<=x+dx<=n-1 and 0<=y+dy<=n-1:
                u=n*y+x
                v=n*(y+dy)+x+dx
                fee=fees[n*(y+dy)+x+dx]
                if fee!=1:
                    g[u].append([v,fee])
                    g[u].append([v+n*n,1])
                    g[u+n*n].append([v+n*n,fee])
                else:
                    g[u].append([v,1])
                    g[u+n*n].append([v+n*n,1])
hq = [(0, 0)]
heapq.heapify(hq)
INF=10**18
cost = [INF]*(2*n*n)
cost[0] = 0
while hq:
    c, v = heapq.heappop(hq)
    if c > cost[v]:
        continue
    for u, d in g[v]:
        tmp = d + cost[v]
        if tmp < cost[u]:
            cost[u] = tmp
            heapq.heappush(hq, (tmp, u))
print(min(cost[n*n-1],cost[2*n*n-1]))