#include <bits/stdc++.h>
using namespace std;
typedef long long   signed int LL;
typedef long long unsigned int LU;
#define incID(i, l, r) for(LL i = (l)    ; i <  (r); ++i)
#define incII(i, l, r) for(LL i = (l)    ; i <= (r); ++i)
#define decID(i, l, r) for(LL i = (r) - 1; i >= (l); --i)
#define decII(i, l, r) for(LL i = (r)    ; i >= (l); --i)
#define inc(i, n)  incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec(i, n)  decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))
#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define  ALL(v)  v.begin(),  v.end()
#define RALL(v) v.rbegin(), v.rend()
template<typename T> bool setmin  (T & a, T b) { if(b <  a) { a = b; return true; } else { return false; } }
template<typename T> bool setmax  (T & a, T b) { if(b >  a) { a = b; return true; } else { return false; } }
template<typename T> bool setmineq(T & a, T b) { if(b <= a) { a = b; return true; } else { return false; } }
template<typename T> bool setmaxeq(T & a, T b) { if(b >= a) { a = b; return true; } else { return false; } }
LL mo(LL a, LL b) { assert(b > 0); a %= b; if(a < 0) { a += b; } return a; }
LL fl(LL a, LL b) { assert(b > 0); return (a > 0 ? a / b : (a - b + 1) / b); }
LL ce(LL a, LL b) { assert(b > 0); return (a < 0 ? a / b : (a + b - 1) / b); }
template<typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
#define bit(b, i) (((b) >> (i)) & 1)
#define BC __builtin_popcountll
#define SC static_cast
#define SI(v) SC<int>(v.size())
#define SL(v) SC<LL >(v.size())
#define RF(e, v) for(auto & e: v)
#define ef else if
#define UR assert(false)

// ---- ----

template<typename T> class SegmentTree {
private:
	int n, s;
	vector<T> a;
	function<T(T &, T &)> f;
	T e;
	bool ok;
public:
	SegmentTree() { n = 0; }
	SegmentTree(int nn, function<T(T &, T &)> ff, T ee) { init(nn, ff, ee); }
	void init(int nn, function<T(T &, T &)> ff, T ee) {
		n = nn;
		f = ff;
		e = ee;
		s = 1;
		while(s < n) { s *= 2; }
		a = vector<T>(2 * s, e);
		ok = true;
	}
	void shift(int & p) {
		assert(inID(p, 0, n));
		p += s;
	}
	void apply(int p, function<void(T &)> g) {
		shift(p);
		g(a[p]);
		while(p > 1) {
			p /= 2;
			a[p] = f(a[2 * p], a[2 * p + 1]);
		}
	}
	T fold_ID(int l, int r) {
		assert(ok);
		assert(inII(l, 0, n)); l += s;
		assert(inII(r, 0, n)); r += s; r--;
		T x = e, y = e;
		while(l <= r) {
			if(l % 2 == 1) { x = f(x, a[l]); l++; }
			if(r % 2 == 0) { y = f(a[r], y); r--; }
			l /= 2;
			r /= 2;
		}
		return f(x, y);
	}
	T fold_II(int l, int r) { return fold_ID(l + 0, r + 1); }
	T fold_CI(int l, int r) { return fold_ID(l + 1, r + 1); }
	T fold_CD(int l, int r) { return fold_ID(l + 1, r + 0); }
	const T & operator[](int p) {
		shift(p);
		return a[p];
	}
	T & ref(int p) {
		shift(p);
		ok = false;
		return a[p];
	}
	void update() {
		dec(i, s) { a[i] = f(a[2 * i], a[2 * i + 1]); }
		ok = true;
	}
};
#define OP(s) [&](auto & A, auto & B) { return s; }
#define AP(s) [&](auto & A) { s; }

// ---- ----

int main() {
	int n, m;
	cin >> n >> m;
	vector<array<LL, 3>> v;
	inc(i, m) {
		LL l, r, c;
		cin >> l >> r >> c;
		l--; r--;
		v.push_back({ l, r, c });
	}
	sort(ALL(v));
	
	LL INF = 1e18;
	SegmentTree<LL> st(n, OP(min(A, B)), INF);
	st.apply(0, AP(A = 0));
	RF(e, v) {
		LL l = e[0];
		LL r = e[1];
		LL c = e[2];
		LL mi = st.fold_II(l, r) + c;
		st.apply(r, AP(setmin(A, mi)));
	}
	
	cout << (st[n - 1] == INF ? -1 : st[n - 1]) << endl;
	
	return 0;
}


Measured memory_bytes: 5104