N,M=map(int,input().split())
LRC=sorted([list(map(int,input().split())) for i in range(M)])
class rmq:
    def __init__(self,N):
        self.N0=2**(N-1).bit_length()
        self.INF=2**31-1
        self.data=[self.INF]*(2*self.N0)
        self.lazy=[None]*(2*self.N0)
    def gindex(self,l, r):
        L = l + self.N0; R = r + self.N0
        lm = (L // (L & -L)) >> 1
        rm = (R // (R & -R)) >> 1
        while L < R:
            if R <= rm:
                yield R
            if L <= lm:
                yield L
            L >>= 1; R >>= 1
        while L:
            yield L
            L >>= 1
    def propagates(self,*ids):
        for i in reversed(ids):
            v = self.lazy[i-1]
            if v is None:
                continue
            self.lazy[2*i-1] = self.data[2*i-1] = self.lazy[2*i] = self.data[2*i] = v
            self.lazy[i-1] = None
    def update(self,l, r, x):
        *ids, = self.gindex(l, r)
        self.propagates(*ids)
        L = self.N0 + l; R = self.N0 + r
        while L < R:
            if R & 1:
                R -= 1
                self.lazy[R-1] = self.data[R-1] = x
            if L & 1:
                self.lazy[L-1] = self.data[L-1] = x
                L += 1
            L >>= 1; R >>= 1
        for i in ids:
            self.data[i-1] = min(self.data[2*i-1], self.data[2*i])
    def rmin(self,l, r):
        self.propagates(*self.gindex(l, r))
        L = self.N0 + l; R = self.N0 + r
        s = self.INF
        while L < R:
            if R & 1:
                R -= 1
                s = min(s, self.data[R-1])
            if L & 1:
                s = min(s, self.data[L-1])
                L += 1
            L >>= 1; R >>= 1
        return s
d=rmq(N)
d.update(0,1,0)
for l,r,c in LRC:
    d.update(r-1,r,min(d.rmin(r-1,r),d.rmin(l-1,r-1)+c))
a=d.rmin(N-1,N)
print(a if a<d.INF else -1)