def main():
    import sys
    input = sys.stdin.readline

    INF = 10 ** 18

    N, M = map(int, input().split())

    G = [[] for _ in range(N + 1)]
    for _ in range(M):
        L, R, C = map(int, input().split())
        G[L].append([C, R])
        G[R].append([C, L])

    for i in range(1, N):
        G[i + 1].append([0, i])

    d = [INF] * (N + 1)

    def dijkstra(s):
        from heapq import heappop, heappush
        d[s] = 0
        pque = []
        heappush(pque, [0, s])
        
        while pque:
            p = heappop(pque)
            v = p[1]
            if d[v] < p[0]:
                continue
            for i in range(len(G[v])):
                e = G[v][i]
                if d[e[1]] > d[v] + e[0]:
                    d[e[1]] = d[v] + e[0]
                    heappush(pque, [d[e[1]], e[1]])

    dijkstra(1)

    print (-1 if d[-1] == INF else d[-1])

if __name__ == '__main__':
    main()