class SegmentTree():
  def __init__(self, n, INF):
    self.n = n
    self.INF = INF
    self.log = (n - 1).bit_length()
    self.size = 1 << self.log
    self.data = [self.INF] * (2 * self.size)
  
  def get(self, p):
    #assert 0 <= p < self.n
    res = self.INF
    p += self.size
    for _ in range(self.log+1):
      res = min(res, self.data[p])
      p >>= 1
    return res

  def set(self, p, x):
    #assert 0 <= p < self.n
    p += self.size
    self.data[p] = x

  def range_set(self, l, r, x):
    #assert 0 <= l <= r <= self.n
    l += self.size
    r += self.size
    while l < r:
      if l & 1:
        self.data[l] = min(self.data[l], x)
        l += 1
      if r & 1:
        r -= 1
        self.data[r] = min(self.data[r], x)
      l >>= 1
      r >>= 1

import sys
input = sys.stdin.readline

N, M = map(int, input().split())
INF = 10**18
lst = SegmentTree(N,INF)
lst.set(0,0)
A = [list(map(int, input().split())) for _ in range(M)]
A.sort(key=lambda x:(x[0],x[1]))
for l,r,c in A:
  lst.range_set(l-1,r,lst.get(l-1)+c)
ans = lst.get(N-1)
print(ans if ans<INF else -1)