import sys
from heapq import heapify, heappop, heappush

sr = lambda: sys.stdin.readline().rstrip()
ir = lambda: int(sr())
lr = lambda: list(map(int, sr().split()))

def dijkstra(start):
    dist = [INF] * (N+1)
    dist[start] = 0
    que = [(0, start)]
    while que:
        d, prev = heappop(que)
        if dist[prev] < d:
            continue
        for cost, next in graph[prev]:
            d1 = d + cost
            if dist[next] > d1:
                dist[next] = d1
                heappush(que, (d1, next))
    return dist

N, M = lr()
INF = 10 ** 15
graph = [[] for _ in range(N+1)] # 1-indexed
for i in range(2, N+1):
    graph[i].append((0, i-1))

for _ in range(M):
    l, r, c = lr()
    graph[l].append((c, r))

dist = dijkstra(1)

if dist[N] == INF:
    print(-1)
else:
    print(dist[N])
# 19