import sys
from heapq import heappush, heappop
input = sys.stdin.readline


def dijkstra(s, n, edge):
    d = [float("inf")] * n
    d[s] = 0
    q = []
    for e in edge[s]:
        heappush(q, e)
    while q:
        minedge = heappop(q)
        v = minedge[1]
        if d[v] < minedge[0]:
            continue
        d[v] = minedge[0]
        for e in edge[v]:
            heappush(q, [e[0]+d[v], e[1]])
    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, N, E)

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


if __name__ == '__main__':
    main()
