import sys
import numpy as np
from scipy.sparse.csgraph import dijkstra
from scipy.sparse import csr_matrix
n, m = [int(i) for i in sys.stdin.readline().split()]
INF = 10**9
graph = np.ones(shape=(n,n)) * INF
for i in range(m):
    l, r, c = [int(i) for i in sys.stdin.readline().split()]
    l -= 1
    r -= 1
    graph[l][:r+1] = np.where(graph[l][:r+1] < c, graph[l][:r+1], np.ones_like(graph[l][:r+1]) * c)
graph = np.where(graph == INF, np.zeros_like(graph), graph)
graph = csr_matrix(graph)
res = dijkstra(graph, indices=[0])
print(int(res[0][-1]) if res[0][-1] < INF else -1)
