class segtree():
    def segfunc(x,y):
        return min(x,y)
    #####単位元######
    ide_ele = -1
    n=1
    init_val=[0]*n
    #num:n以上の最小の2のべき乗
    num =2**(n-1).bit_length()
    seg=[ide_ele]*2*num
    
    def __init__(self,INIT_VAL,SEGFUNC,IDE_ELE):
        self.ide_ele=IDE_ELE
        self.init_val=[i for i in INIT_VAL]
        self.segfunc=SEGFUNC
        self.n=len(self.init_val)
        self.num =2**(self.n-1).bit_length()
        self.seg=[self.ide_ele]*2*self.num

        #set_val
        for i in range(self.n):
            self.seg[i+self.num-1]=self.init_val[i]    
        #built
        for i in range(self.num-2,-1,-1) :
            self.seg[i]=self.segfunc(self.seg[2*i+1],self.seg[2*i+2]) 
        
    def update(self,k,x):
        k += self.num-1
        self.seg[k] = x
        while k+1:
            k = (k-1)//2
            self.seg[k] = self.segfunc(self.seg[k*2+1],self.seg[k*2+2])
        
    def query(self,p,q):
        if q<=p:
            return self.ide_ele
        p += self.num-1
        q += self.num-2
        res=self.ide_ele
        while q-p>1:
            if p&1 == 0:
                res = self.segfunc(res,self.seg[p])
            if q&1 == 1:
                res = self.segfunc(res,self.seg[q])
                q -= 1
            p = p//2
            q = (q-1)//2
        if p == q:
            res = self.segfunc(res,self.seg[p])
        else:
            res = self.segfunc(self.segfunc(res,self.seg[p]),self.seg[q])
        return res
N,M=map(int,input().split())
Edge=[]
for i in range(M):
    l,r,c=map(int,input().split())
    Edge.append((l-1,r-1,c))
Edge.sort()
INF=10**15
dp=segtree([INF for i in range(N)],min,INF)
dp.update(0,0)
for i in range(M):
    l,r,c=Edge[i]
    dist=dp.query(l,r+1)
    tmp=dp.query(r,r+1)
    dp.update(r,min(tmp,dist+c))
ans=dp.query(N-1,N)
if ans==INF:
    ans=-1
print(ans)
