#include "bits/stdc++.h"
using namespace std;

#define int long long
typedef pair<int, int> P;
//#define mod 1000000007
#define INF (1LL<<60)

#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define YES puts("YES")
#define Yes puts("Yes")
#define NO  puts("NO")
#define No  puts("No")
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }

const int dx[4] = { 1, 0, -1, 0 };
const int dy[4] = { 0, 1, 0, -1 };


#if 1
//-------------
// DUMPマクロ
// https://www.creativ.xyz/dump-cpp-652/

// vector
template <typename T>
istream & operator>>(istream & is, vector<T> & vec) {
	for (T& x : vec) is >> x;
	return is;
}
// pair
template <typename T, typename U>
ostream& operator<<(ostream& os, pair<T, U>& pair_var) {
	os << "(" << pair_var.first << ", " << pair_var.second << ")";
	return os;
}
// vector
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
	os << "{";
	for (int i = 0; i < vec.size(); i++) {
		os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
	}
	os << "}";
	return os;
}
// map
template <typename T, typename U>
ostream& operator<<(ostream& os, map<T, U>& map_var) {
	os << "{";
	repi(itr, map_var) {
		os << *itr;
		itr++;
		if (itr != map_var.end()) os << ", ";
		itr--;
	}
	os << "}";
	return os;
}
// set
template <typename T>
ostream& operator<<(ostream& os, set<T>& set_var) {
	os << "{";
	repi(itr, set_var) {
		os << *itr;
		itr++;
		if (itr != set_var.end()) os << ", ";
		itr--;
	}
	os << "}";
	return os;
}

#define DUMPOUT cerr

void dump_func() {
	DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head&& head, Tail&& ... tail) {
	DUMPOUT << head;
	if (sizeof...(Tail) > 0) {
		DUMPOUT << ", ";
	}
	dump_func(std::move(tail)...);
}
#ifdef _DEBUG
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif
#endif

struct Edge {
	int to;     // 辺の行き先
	int weight; // 辺の重み
	Edge(int t, int w) : to(t), weight(w) { }
};

using Graph = vector<vector<Edge>>;

struct node {
	int v, d;
	node(int v, int d) :v(v), d(d) {}
	bool operator<(const node& n) const { return d > n.d; }
};


Graph G;
vector<int> dist;

void dijkstra(int s) {
	dist.assign(G.size(), -INF);

	priority_queue<node> que;
	que.push(node(s, 0));
	while (!que.empty()) {
		int v = que.top().v, d = que.top().d;
		que.pop();
		if (dist[v] >= 0 and d >= dist[v]) continue;
		dist[v] = d;
		for (Edge& e : G[v]) {
			que.push(node(e.to, d + e.weight));
		}
	}
}
signed main() {

	int N, M;
	cin >> N >> M;
	G.assign(N, { {} });
	rep(i, M) {
		int l, r, c;
		cin >> l >> r >> c;
		l--; r--;
		G[l].push_back({ r,c });
	}
	rep(i, N - 1) {
		G[i + 1].push_back({ i,0 });
	}
	dijkstra(0);

	if (dist[N - 1] == -INF) cout << -1 << endl;
	else cout << dist[N - 1] << endl;

	return 0;
}


Measured memory_bytes: 10100