# -*- 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):
    prev = {}
    q = []
    d = defaultdict(lambda: INF)
    d[s] = 0
    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()


### FIXED CODE ###

from collections import defaultdict
from heapq import heappop, heappush


class Graph():

    def __init__(self):
        self.graph = defaultdict(list)

    def __len__(self):
        return len(self.graph)

    def add_edge(self, src, dst, weight=1):
        self.graph[src].append((dst, weight))

    def get_nodes(self):
        return self.graph.keys()


class Dijkstra():

    def __init__(self, graph, start):
        self.g = graph.graph

        self.dist = defaultdict(lambda: float("inf"))
        self.dist[start] = 0

        self.prev = defaultdict(lambda: None)

        self.Q = []
        heappush(self.Q, (self.dist[start], start))

        while self.Q:
            dist_u, u = heappop(self.Q)

            if self.dist[u] < dist_u:
                continue

            for v, weight in self.g[u]:
                alt = dist_u + weight
                if self.dist[v] > alt:
                    self.dist[v] = alt
                    self.prev[v] = u
                    heappush(self.Q, (alt, v))

    def shortest_distance(self, goal):

        return self.dist[goal]

    def shortest_path(self, goal):
        path = []
        node = goal

        while node is not None:
            path.append(node)
            node = self.prev[node]

        return path[::-1]


import sys
input=sys.stdin.readline
n,m=map(int,input().split())
g=Graph()

for _ in range(m):
    l,r,c=map(int,input().split())
    g.add_edge(l,r,c)
for i in range(2,n+1):
    g.add_edge(i,i-1,0)


d = Dijkstra(g,1)
ans=d.shortest_distance(n)
print(ans if ans!=float("inf") else -1)