# -*- coding: utf-8 -*-
import sys
from collections import defaultdict
from operator import itemgetter
from heapq import heappop, heappush
input = sys.stdin.readline
def inpl(): return list(map(int, input().split()))

N, M = inpl()
D = [0] + [10**18]*(N-1)
g0 = itemgetter(0)
Q = sorted([inpl() for _ in range(M)], key=g0, reverse=True)
#print(*Q, sep="\n")

HQ = [(0, 0)]

for i in range(N):
    while HQ[0][1] < i:
        heappop(HQ)
    v = HQ[0][0]
    while len(Q) and (i == (Q[-1][0]-1)):
        l, r, c = Q.pop()
        heappush(HQ, (v+c, r-1))


if v == 10**18:
    print(-1)
else:
    print(v)