import sys
input = sys.stdin.readline


def main():


    N, M = map(int, input().split())
    LRC = [list(map(int, input().split())) for _ in range(M)]

    LRC.sort(key=lambda x: (x[0], x[1]))
    # print(LRC)

    dp = [10 ** 9 + 10] * (N + 1)
    dp[1] = 0

    sml, big = LRC[0][0], LRC[0][1]

    for i in range(1, M):
        l, r, c = LRC[i]
        if l <= big:
            if big <= r:
                big = r
        else:
            print(-1)
            exit()
    if sml != 1 or big != N:
        print(0)
        exit()


    for lrc in LRC:
        l, r, c = lrc
        for i in range(l, r + 1):
            dp[i] = min(dp[i], dp[l] + c)
    print(dp[-1])


if __name__ == '__main__':
    main()
