from collections import deque
from collections import defaultdict

N, M = map(int, input().split())
d = defaultdict(int)
s = set()
for i in range(M):
    h, w, c = map(int, input().split())
    h-=1;w-=1
    s.add((h, w))
    d[str(h)+'_'+str(w)] = c
cost = [[[float('inf'), 0] for _ in range(N)] for _ in range(N)]
cost[0][0][0] = 0
q = deque()
q.append((0, 0))
xx = [0,1,0,-1]
yy = [1,0,-1,0]
while q:
    x, y = q.popleft()
    for px, py in zip(xx, yy):
        cx = x+px
        cy = y+py
        if 0<=cx<N and 0<=cy<N:
            if (cx, cy) in s:
                c = d[str(cx)+'_'+str(cy)]
                if cost[cx][cy][0]-cost[cx][cy][1]>cost[x][y][0]+1+c-max(cost[x][y][1], c):
                    cost[cx][cy][0] = cost[x][y][0]+1+c
                    cost[cx][cy][1] = max(cost[x][y][1], c)
                    q.append((cx, cy))
            else:
                if cost[cx][cy][0]-cost[cx][cy][1]>cost[x][y][0]+1-cost[x][y][1]:
                    cost[cx][cy][0] = cost[x][y][0]+1
                    cost[cx][cy][1] = cost[x][y][1]
                    q.append((cx, cy))
print(cost[-1][-1][0]-cost[-1][-1][1])