import sys
import heapq
input = sys.stdin.readline


def dijkstra(s, E):
    inf = float('inf')
    d = [inf for i in range(len(E))]
    d[s] = 0
    q = [(0, s)]
    heapq.heapify(q)
    while q:
        c, p = heapq.heappop(q)
        if d[p] < c:
            continue
        for cost, to in E[p]:
            if d[to] > d[p] + cost:
                d[to] = d[p] + cost
                heapq.heappush(q, (d[to], to))
    return d


def main():
    N, M = map(int, input().split())
    E = [[] for _ in range(N)]
    for _ in range(M):
        s, t, c = map(int, input().split())
        E[s-1].append([c, t-1])
    for i in range(N-1):
        E[i+1].append([0, i])

    A = dijkstra(0, E)

    ans = A[N-1]
    if ans == float("inf"):
        print(-1)
    else:
        print(ans)


if __name__ == '__main__':
    main()
