import sys
import heapq
sys.setrecursionlimit(10**7)
n, m = [int(i) for i in sys.stdin.readline().split()]
INF = 10**9
graph = {j:[(j-1, 1e-10)] if j > 0 else [] for j in range(n)}
for i in range(m):
    l, r, c = [int(i) for i in sys.stdin.readline().split()]
    l -= 1
    r -= 1
    graph[l].append((r, c))
    graph[r].append((l, c))
que = [(0,0)]
already = set()
res = [-1 for i in range(n)]
while len(que) > 0:
    cost, cand = heapq.heappop(que)
    if cand in already:
        continue
    res[cand] = cost
    already.add(cand)
    for _next, _next_cost in graph[cand]:
        if _next in already:
            continue
        heapq.heappush(que, (cost + _next_cost, _next))
print(int(res[-1]))
