def dijkstra(s,n,w,cost):
    d = [float("inf")] * n
    used = [False] * n
    d[s] = 0
    
    while True:
        v = -1
        for i in range(n):
            if (not used[i]) and (v == -1):
               v = i
            elif (not used[i]) and d[i] < d[v]:
                v = i
        if v == -1:
               break
        used[v] = True
               
        for j in range(n):
               d[j] = min(d[j],d[v]+cost[v][j])
    return d

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

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