#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

#define int long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define all(c) begin(c),end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1000000007;
template<class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

template<typename T>
vector<T> gen_v(size_t a) { return vector<T>(a); }
template<typename T, typename ...Ts>
auto gen_v(size_t a, Ts... ts) { return vector<decltype(gen_v<T>(ts...))>(a, gen_v<T>(ts...)); }
template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T &t, const V &v) { t = v; }
template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T &t, const V &v) { for (auto &e : t) fill_v(e, v); }

template<class T, class ...Tail>
void tiedSort(vector<T> &a, vector<Tail>&... tail) {
	int n = a.size();
	using S = tuple<T, Tail...>;
	vector<S> s(n);
	for (int i = 0; i < n; i++)
		s[i] = make_tuple(a[i], tail[i]...);
	sort(s.begin(), s.end());
	for (int i = 0; i < n; i++)
		tie(a[i], tail[i]...) = s[i];
}

struct RangeUpdateQuery {
	int n;
	vector<int> d;
	vector<int> timestamp;
	int time;
	RangeUpdateQuery(int m) {
		for (n = 1; n < m; n *= 2);
		d.assign(n * 2, INF);
		time = 0;
		timestamp.assign(n * 2, 0);
	}
	void update(int l, int r, int x) {
		time++;
		for (; l && l + (l&-l) <= r; l += l & -l) {
			d[(n + l) / (l&-l)] = x;
			timestamp[(n + l) / (l&-l)] = time;
		}
		for (; l < r; r -= r & -r) {
			d[(n + r) / (r&-r) - 1] = x;
			timestamp[(n + r) / (r&-r) - 1] = time;
		}
	}
	int get(int i) {
		int ret = d[n + i];
		int max_time = timestamp[n + i];
		for (int j = (n + i) / 2; j > 0; j >>= 1) {
			if (max_time > timestamp[j])continue;
			max_time = timestamp[j];
			ret = d[j];
		}
		return ret;
	}
};

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N, M; cin >> N >> M;
	vector<int> L(M), R(M), C(M); rep(i, 0, M) {
		cin >> L[i] >> R[i] >> C[i];
		L[i]--;
	}

	tiedSort(L, R, C);

	RangeUpdateQuery ruq(N);
	ruq.update(0, 1, 0);

	rep(i, 0, M) {
		dump(i, L[i], R[i], C[i]);
		int mini = ruq.get(L[i]);
		if (mini == INF) {
			cout << -1 << endl;
			return 0;
		}
		int c = mini + C[i];

		auto f = [&](int x) {
			return ruq.get(x) > c;
		};
		auto binarySearch = [&](int ng, int ok) {
			if (f(ng))return ng;
			while (ng + 1 < ok) {
				int m = (ng + ok) / 2;
				if (f(m))
					ok = m;
				else
					ng = m;
			}
			return ok;
		};

		int l = binarySearch(L[i], N);
		dump(l);
		dump(ruq.get(l));

		vector<int> tmp(N);
		rep(i, 0, N) {
			tmp[i] = ruq.get(i);
		}
		dump(tmp);

		ruq.update(l, R[i], c);

	}
	int ans = ruq.get(N - 1);
	if (ans == INF)ans = -1;
	cout << ans << endl;

	return 0;
}