# -*- coding: utf-8 -*-
import bisect
import heapq
import math
import random
import sys
from collections import Counter, defaultdict, deque
from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal
from functools import lru_cache, reduce
from itertools import combinations, combinations_with_replacement, product, permutations
from operator import add, mul, sub

sys.setrecursionlimit(100000)
input = sys.stdin.readline
INF = 2**62-1

def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input().strip()


def read_str_n():
    return list(map(str, input().split()))


def error_print(*args):
    print(*args, file=sys.stderr)


def mt(f):
    import time

    def wrap(*args, **kwargs):
        s = time.time()
        ret = f(*args, **kwargs)
        e = time.time()

        error_print(e - s, 'sec')
        return ret

    return wrap


def Dijkstra(g, s, N):
    V = set(range(1, N+1))

    d = {}
    prev = {}
    q = []
    for v in V:
        if v == s:
            d[v] = 0
        else:
            d[v] = INF
    heapq.heappush(q, (d[s], s))

    while q:
        c, u = heapq.heappop(q)
        if d[u] < c:
            continue
        for v in g[u]:
            alt = g[u][v] + c
            if d[v] > alt:
                d[v] = alt
                prev[v] = u
                heapq.heappush(q, (alt, v))
    return d


@mt
def slv(N, LRC):
    g = defaultdict(dict)

    for l, r, c in LRC:
        g[l][r] = c
    
    for i in range(1, N):
        g[i+1][i] = 0

    
    d = Dijkstra(g, 1, N)

    return -1 if d[N] == INF else d[N]


def main():
    N, M = read_int_n()
    LRC = [read_int_n() for _ in range(M)]
    print(slv(N, LRC))


if __name__ == '__main__':
    main()
