from heapq import heappush,heappop
n,m=map(int,input().split())
a=[[[i-1,0]]for i in range(n+1)]
a[0]=[]
for i in range(m):
    l,r,c=map(int,input().split())
    a[l].append([r,c])
q=[float("INF")]*(n+1)
q[1]=0
c=[]
heappush(c,[0,1])
while c:
    s,t=heappop(c)
    if t==n:
        print(s)
        break
    for i,j in a[t]:
        if q[i]>s+j:
            q[i]=s+j
            heappush(c,[s+j,i])
else:
    print(-1)