import numpy as np
import heapq

INF=10**12
def daik(s,g,d,adj):
    used=np.array([])
    heap=[]
    table=np.array([INF for i in range(N)])
    table[s]=0
    heapq.heappush(heap,(table[s],s))
    used=np.append(used,s)
                
    while heap and not(g in used):
        q=heapq.heappop(heap)
        a=q[1]
        used=np.append(used,a)
        for i in adj[a]:
            if not(i in used):
                cost=d[a,i] 
                if cost!=INF:
                    table[i]=min(table[i],table[a]+cost)
                    heapq.heappush(heap,(table[i],i))
    return table[g]

N,M=map(int,input().split())
LRC=[]
for i in range(M):
    L,R,C=map(int,input().split())
    LRC.append([L-1,R-1,C])
LRC=np.array(LRC)
LRC=LRC[np.argsort(LRC[:,2])][::-1]

d=np.array([[INF for j in range(N)] for i in range(N)])
adj=[[] for i in range(N)]
print(adj)
for i in range(N-1):
    d[i+1,i]=0
    adj[i+1].append(i)
    adj[i].append(i+1)
for i in range(M):
    C=LRC[i,2]
    d[LRC[i,0],LRC[i,1]]=C
    d[LRC[i,1],LRC[i,0]]=C
    adj[LRC[i,0]].append(LRC[i,1])
    adj[LRC[i,1]].append(LRC[i,0])
adj=np.unique(adj,axis=1)
print(adj)
ans=daik(0,N-1,d,adj)
if ans==INF:
    print(-1)
else:
    print(ans)