import sys
input = sys.stdin.readline
N,M = map(int,input().split())
LRC = [tuple(map(int,input().split())) for i in range(M)]
LRC.sort()

INF = float('inf')
class SegTreeMin:
    def __init__(self,arr):
        n = 1
        while n < len(arr):
            n *= 2
        self.n = n
        nodes = [INF]*(2*n-1)
        nodes[n-1:n-1+len(arr)] = arr
        for i in range(n-2,-1,-1):
            nodes[i] = min(nodes[2*i+1],nodes[2*i+2])
        self.nodes = nodes

    def update(self,i,val):
        i += self.n-1
        self.nodes[i] = val
        while i:
            i = (i-1)//2
            self.nodes[i] = min(self.nodes[2*i+1],self.nodes[2*i+2])

    def query(self,a,b,k=0,l=0,r=-1):
        if r < 0: r = self.n
        if r <= a or b <= l: return INF
        if a <= l and r <= b: return self.nodes[k]
        return min(self.query(a,b,2*k+1,l,(l+r)//2), self.query(a,b,2*k+2,(l+r)//2,r))

segt = SegTreeMin([0] + [INF]*N)
for l,r,c in LRC:
    mn = segt.query(l-1,r-1)
    org = segt.query(r-1,r)
    if org > mn+c:
        segt.update(r-1, mn+c)

ans = segt.query(N-1,N)
print(-1 if ans==INF else ans)