#include <bits/stdc++.h>
using namespace std;
#define Rep(i,n) for(int i=0;i<(int)(n);i++)
#define For(i,n1,n2) for(int i=(int)(n1);i<(int)(n2);i++)
#define REP(i,n) for(ll i=0;i<(ll)(n);i++)
#define RREP(i,n) for(ll i=((ll)(n)-1);i>=0;i--)
#define FOR(i,n1,n2) for(ll i=(ll)(n1);i<(ll)(n2);i++)
#define RFOR(i,n1,n2) for(ll i=((ll)(n1)-1);i>=(ll)(n2);i--)
#define all(a)  (a).begin(),(a).end()
#define SORT(a) sort((a).begin(),(a).end())
#define oorret 0
#define oor(x) [&](){try{x;} catch(const out_of_range& oor){return oorret;} return x;}()
#define IOS cin.tie(0),ios::sync_with_stdio(false)
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
template<typename T1, typename T2> inline bool chmin(T1& a, T2 b) { if (a > b) { a = b; return 1; }return 0; }
template<typename T1, typename T2> inline bool chmax(T1& a, T2 b) { if (a < b) { a = b; return 1; }return 0; }
template<class Type>struct is_vector : std::false_type {};
template<class ValueType, class Alloc>struct is_vector<std::vector<ValueType, Alloc>> : std::true_type {};
template <typename T> inline ostream& operator << (ostream& out, const vector<T>& v) {
	if (v.empty())return out;
	constexpr bool is_vector_v = is_vector<T>::value;
	if (is_vector_v)for (auto itr = v.begin(); itr != v.end();)out << (*itr), out << ((++itr != v.end()) ? "\n" : "");
	else for (auto itr = v.begin(); itr != v.end();)out << (*itr), out << ((++itr != v.end()) ? " " : "");
	return out;
}
inline void put() {}
template<class T> inline void put(const T& first) { std::cout << first << "\n"; }
template<class T, class... N> inline void put(const T& first, const N& ... rest) { std::cout << first << " "; put(rest...); }
inline void putn() {}
template<class T, class... N> inline void putn(const T& first, const N& ... rest) { std::cout << first << "\n"; putn(rest...); }

template<typename T>
T upmin(const T& l, const T& r) {//SegmentTreeのupdate関数
	T res;
	res = min(l,r);
	return res;
}

template<typename T>
class SegmentTree {
public:
	using F = function<T(T&, T&)>;
	vector<T> seg;
	int sz = 1;
	T unit;
	F up;
	SegmentTree(int n, F up, T unit = 0) :up(up), unit(unit) {
		while (sz < n) sz <<= 1;
		seg.resize(sz * 2, unit);
	}
	void set(const int& k, const T& x) {//左からk番目の葉にxを代入する
		seg[sz - 1 + k] = x;
	}
	T get(const int& k) {//左からk番目の葉を得る
		return seg[sz - 1 + k];
	}
	void update(int k, T v) {// k 番目の値をup(seg[k],v)にする
		k += sz - 1;
		seg[k] = up(seg[k], v);
		while (k > 0) {
			k = (k - 1) / 2;
			seg[k] = up(seg[2 * k + 1], seg[2 * k + 2]);
		}
	}
	T query(int a, int b) {//[a,b)のupの合成を求める
		int k = 0, l = 0, r = sz;
		if (r <= a || b <= l) {
			return unit;
		}
		if (a <= l && r <= b) {
			return seg[k];
		} else {
			T vl = query(a, b, k * 2 + 1, l, (l + r) / 2);
			T vr = query(a, b, k * 2 + 2, (l + r) / 2, r);
			return up(vl, vr);
		}
	}
	T query(int a, int b, int k, int l, int r) {//[a,b)のupの合成を求める(a,b,0,0,sz)のように使う
		if (r <= a || b <= l) {
			return unit;
		}
		if (a <= l && r <= b) {
			return seg[k];
		} else {
			T vl = query(a, b, k * 2 + 1, l, (l + r) / 2);
			T vr = query(a, b, k * 2 + 2, (l + r) / 2, r);
			return up(vl, vr);
		}
	}
};

int main() {
	IOS;
	int n, m;
	cin >> n >> m;
	vector<ll> l(m), r(m), c(m);
	using pi = pair<pair<ll, ll>, ll>;
	vector<pi> p(m);
	REP(i, m) {
		cin >> l[i] >> r[i] >> c[i];
		l[i]--;
		r[i]--;
		p[i] = pi(P(l[i], r[i]), c[i]);
	}
	sort(all(p));
	SegmentTree<ll> sg(n, upmin<ll>, LLONG_MAX);
	sg.update(0, 0);
	REP(i, m) {
		int l = p[i].first.first;
		int r = p[i].first.second;
		ll c = p[i].second;
		ll t = sg.query(l, r);
		if (t != LLONG_MAX) {
			sg.update(r, t + c);
		}
	}
	ll res = sg.get(n - 1);
	if (res != LLONG_MAX) {
		put(res);
	} else {
		put(-1);
	}
	return 0;
}


Measured memory_bytes: 6912