import heapq

n, m = [int(v) for v in input().split(' ')]

links = []
for i in range(n):
    links.append([])

for i in range(m):
    l, r, c = [int(v) for v in input().split(' ')]
    l -= 1
    r -= 1
    links[r].append((l, c))

h = []
heapq.heappush(h, (0, n-1))
closed = 0
while len(h) > 0:
    c0, n0 = heapq.heappop(h)
    closed |= (1 << n0)
    if n0 == 0:
        print(c0)
        exit()

    for n1, c1 in links[n0]:
        if (closed & (1 << n1)) != 0:
            continue
        heapq.heappush(h, (c0 + c1, n1))

    n2 = n0 + 1
    if n2 < n:
        if (closed & (1 << n2)) == 0:
            heapq.heappush(h, (c0, n0 + 1))

print("-1")