N, M =map(int, input().split())
route_list  = [[float("inf") for i in range(N)] for i in range(N)] 

for i in range(M):
    l,r,c = map(int, input().split())
    for k in range(l,r):
      for j in range(k+1,r+1):

        if route_list[k-1][j-1] == 0:
          route_list[k-1][j-1] = c
          route_list[j-1][k-1] = c
        else:
          route_list[k-1][j-1] = min(route_list[k-1][j-1],c)
          route_list[j-1][k-1] = min(route_list[j-1][k-1],c)
def dijkstra(s,n,w,cost):
    #始点sから各頂点への最短距離
    #n:頂点数,　w:辺の数, cost[u][v] : 辺uvのコスト(存在しないときはinf)
    d = [float("inf")] * n
    used = [False] * n
    d[s] = 0
    
    while True:
        v = -1
        #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
        for i in range(n):
            if (not used[i]) and (v == -1):
               v = i
            elif (not used[i]) and d[i] < d[v]:
                v = i
        if v == -1:
               break
        used[v] = True
               
        for j in range(n):
               d[j] = min(d[j],d[v]+cost[v][j])
    return d



ans = dijkstra(0,N,M,route_list)[-1]
if ans ==float('inf'):
  print(-1)
else:
  print(ans)