import heapq
import sys
input=sys.stdin.readline

def calc():
    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**15
    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]))
calc()