def dijkstra(s,n,w,cost):
    #始点sから各頂点への最短距離
    #n:頂点数,　w:辺の数, cost[u][v] : 辺uvのコスト(存在しないときはinf)
    d = [float("inf")] * n
    used = [False] * n
    d[s] = 0
    
    while True:
        v = -1
        #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
        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 = [int(x) for x in input().split()]

cost = [[float("inf") for i in range(n)] for i in range(n)] 
#cost[u][v] : 辺uvのコスト(存在しないときはinf この場合は10**10)
for i in range(m):
    l,r,z = map(int,input().split())
    for j in range(l, r):
        for k in range(j+1, r+1):
            cost[j-1][k-1] = min(z, cost[j-1][k-1])
            cost[k-1][j-1] = min(z, cost[k-1][j-1])
ret = dijkstra(0,n,m,cost)
if (ret[-1] == 'inf'):
    print(-1)
else:
    print(ret[-1])
