import heapq

n, m = map(int, input().split())
edge = [[]] + [[(0, i)] for i in range(n-1)]
for _ in range(m):
  l, r, c = map(int, input().split())
  edge[l-1].append((c, r-1))

inf = 1 << 60
dp = [0] + [inf] * (n - 1)
queue = [(0, 0)]
check = [False] * n
while queue:
  dist, act = heapq.heappop(queue)
  check[act] = True
  for cost, vertex in edge[act]:
    if check[vertex]:
      continue
    d = dist + cost
    if dp[vertex] > dist:
      heapq.heappush(queue, (d, vertex))
      dp[vertex] = d
if dp[-1] < inf:
  print(dp[-1])
else:
  print(-1)