N,M=map(int,input().split())

import heapq
def dijkstra_heap(s):
  #始点sから各頂点への最短距離と直前の点
  dist=[float("inf")]*size_v
  prev=[-1]*size_v
  used=[False]*size_v
  #used=set()
  dist[s]=0
  used[s]=True
  #used.add(s)

  hq_edge=[]
  for v,w in graph[s]:
    heapq.heappush(hq_edge,(w,v,s))

  while hq_edge:
    min_w,min_v,u=heapq.heappop(hq_edge)
    #まだ使われてない頂点の中から最小の距離のものを探す
    if used[min_v]:
    #if min_v in used:
      continue

    dist[min_v]=min_w
    prev[min_v]=u
    used[min_v]=True
    #used.add(min_v)
    for v,w in graph[min_v]:
      if not used[v]: 
      #if not v in used:
        heapq.heappush(hq_edge,(min_w+w,v,min_v))

  return dist,prev

size_v=N+1
s=1

graph=[[] for _ in range(size_v)]
for i in range(2,N):
  graph[i].append((i-1,0))
for _ in range(M):
  L,R,C=map(int,input().split())
  graph[L].append((R,C))
  
dist,prev=dijkstra_heap(s)
#print(dist)
if dist[N]==float("inf"):
  print(-1)
else:
  print(dist[N])