import heapq
import sys

input = sys.stdin.readline

def dijkstra_heap(s,edge):
    #始点sから各頂点への最短距離
    d = [float("inf")] * n
    used = [True] * n #True:未確定
    d[s] = 0
    used[s] = False
    edgelist = []
    for a,b in edge[s]:
        heapq.heappush(edgelist,a*(10**6)+b)
    while len(edgelist):
        minedge = heapq.heappop(edgelist)
        #まだ使われてない頂点の中から最小の距離のものを探す
        if not used[minedge%(10**6)]:
            continue
        v = minedge%(10**6)
        d[v] = minedge//(10**6)
        used[v] = False
        for e in edge[v]:
            if used[e[1]]:
                heapq.heappush(edgelist,(e[0]+d[v])*(10**6)+e[1])
    return d
#########################
n,w = map(int,input().split())
edge = [[] for i in range(n)]
#隣接リスト edge[i]:[コスト,行先]
for i in range(n-1):
    edge[i+1].append([0, i])

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

