import heapq
def dijkstra_heap(s):
    d = [float("inf")] * n
    used = [True] * n 
    d[s] = 0
    used[s] = False
    edgelist = []
    for e in edge[s]:
        heapq.heappush(edgelist,e)
    while len(edgelist):
        minedge = heapq.heappop(edgelist)
        if not used[minedge[1]]:
            continue
        v = minedge[1]
        d[v] = minedge[0]
        used[v] = False
        for e in edge[v]:
            if used[e[1]]:
                heapq.heappush(edgelist,[e[0]+d[v],e[1]])
    return d


n,m = map(int,input().split())

edge = [[] for i in range(n)]
for i in range(m):
    x,y,z = map(int,input().split())
    edge[x-1].append([z,y-1])
    edge[y-1].append([z,x-1]) 
for i in range(1,n):
  edge[i].append([0,i-1])
x=dijkstra_heap(0)[n-1]
if x==float("inf"):
  print(-1)
else:
  print(x)