#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <vector>

#define MOD 1000000007
#define int long long
//#define PI 3.14159265358979

#define rep(i, n) for (int i = 0; i < (int)(n); i++)

using namespace std;

template < typename T >
ostream &operator<<(ostream &os, const vector< T > &A) {
	for (int i = 0; i < A.size(); i++)
		os << A[i] << " ";
	os << endl;
	return os;
}
template <>
ostream &operator<<(ostream &os, const vector< vector< int > > &A) {
	int N = A.size();
	int M;
	if (N > 0)
		M = A[0].size();
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < M; j++)
			os << A[i][j] << " ";
		os << endl;
	}
	return os;
}

typedef pair< int, int > pii;
typedef long long ll;

struct edge {
	int from, to, d, c;
	edge(int _from = 0, int _to = 0, int _d = 0, int _c = 0) {
		from = _from;
		to = _to;
		d = _d;
		c = _c;
	}
	bool operator<(const edge &rhs) const {
		return (d == rhs.d) ? (c < rhs.c) : (d < rhs.d);
	}
};

typedef vector< edge > edges;
typedef vector< edges > graph;
struct flow {
	int to, cap, rev, cost;
	flow(int to = 0, int cap = 0, int rev = 0, int cost = 0) : to(to), cap(cap), rev(rev), cost(cost) {}
};
typedef vector< vector< flow > > flows;

const int di[4] = {0, -1, 0, 1};
const int dj[4] = {-1, 0, 1, 0};
const int ci[5] = {0, 0, -1, 0, 1};
const int cj[5] = {0, -1, 0, 1, 0};
const ll LINF = LLONG_MAX / 2;
const int INF = INT_MAX / 2;
const double PI = acos(-1);

template < typename T, typename U >
bool chmin(T &x, const U &y) {
	if (x > y) {
		x = y;
		return true;
	}
	return false;
}
template < typename T, typename U >
bool chmax(T &x, const U &y) {
	if (x < y) {
		x = y;
		return true;
	}
	return false;
}

struct initializer {
	initializer() {
		cout << fixed << setprecision(11);
	}
};
initializer _____;

struct SegT {
	vector< ll > container;
	int n;
	void update(int i, ll x) {
		int ii = i + n - 1;
		container[ii] = x;
		while (ii > 0) {
			ii = (ii - 1) / 2;
			//////////////////  update to better one  ////////////////////////
			container[ii] = min(container[2 * ii + 1], container[2 * ii + 2]);
		}
	}
	ll get(int l, int r, int l2 = 0, int r2 = -1, int k = 0) {
		if (r2 == -1)
			r2 = n;
		/////////////////  return worst  //////////////////
		if (r2 <= l || r <= l2)
			return LINF;

		if (l <= l2 && r2 <= r)
			return container[k];
		ll vl = get(l, r, l2, (l2 + r2) / 2, 2 * k + 1);
		ll vr = get(l, r, (l2 + r2) / 2, r2, 2 * k + 2);
		/////////////  return better one  //////////////
		return min(vl, vr);
	}
	SegT(vector< int > &A) {
		n = pow(2, (int)log2(A.size()) + 1);
		container = vector< ll >(2 * n - 1, 0);
		for (int i = 0; i < A.size(); i++)
			update(i, A[i]);
	}
};

struct range {
	int l, r, c;
	range(int l = 0, int r = 0, int c = 0) : l(l), r(r), c(c) {}
	bool operator<(const range &rhs) const {
		return l < rhs.l;
	}
};
int N, M, K, T, Q;
signed main() {
	cin >> N >> M;
	vector< range > R(M);
	rep(i, M) cin >> R[i].l >> R[i].r >> R[i].c;
	sort(R.begin(), R.end());
	vector< int > X(N + 1, LINF);
	X[1] = 0;
	SegT st(X);
	rep(i, M) {
		range r = R[i];
		int tmp = st.get(r.l, r.r) + r.c;
		if (tmp < st.get(r.r, r.r + 1))
			st.update(r.r, st.get(r.l, r.r) + r.c);
	}
	int ans = (st.get(N, N + 1) == LINF ? -1 : st.get(N, N + 1));
	//cout << st.container;
	cout << ans << endl;
	return 0;
}

Measured memory_bytes: 5376