import sys
input = sys.stdin.readline

import heapq
INF = 2**31-1

def dijkstra(start, edge):
    n = len(edge)
    dist = [INF]*n
    dist[start] = 0
    que = [(0, start)]
    while que:
        d, v = heapq.heappop(que)
        if dist[v] < d:
            continue
        for nv, nd in edge[v]:
            if dist[nv] > d + nd:
                dist[nv] = d + nd
                heapq.heappush(que, (dist[nv], nv))
    return dist
  
n, m = map(int, input().split())
G = [[] for _ in range(n)]
for i in range(1, n):
  G[i].append((i-1, 0))
for _ in range(m):
  l, r, c = map(int, input().split())
  G[l-1].append((r-1, c))

D = dijkstra(0, G)
ans = D[n-1]
if ans == INF:
  ans = -1
print(ans)