from heapq import heappop, heappush
from collections import defaultdict

INF = 10**20
N,M = map(int,input().split(' '))
g = defaultdict(list)
for i in range(2,N+1):
    g[i].append((i-1,0))
for i in range(1,M+1):
    l,r,c = list(map(int,input().split(' ')))
    g[l].append((r,c))

d = [INF] * (N+1)
d[1] = 0
que = []
heappush(que,(1,0))
while que:
    s, dist = heappop(que)
    if d[s] < dist:
        continue
    for t,c in g[s]:
        if d[t] > d[s] + c:
            d[t] = d[s] + c
            heappush(que,(t,d[t]))
if d[N] == INF:
    print(-1)
    exit()
print(d[N])
        

