#ifdef LOCAL111
	#define _GLIBCXX_DEBUG
#else
	#pragma GCC optimize ("O3")
	#define NDEBUG
#endif
// #define _USE_MATH_DEFINES
#include <bits/stdc++.h>
const long long INF = 1e17;
using namespace std;
template<typename T, typename U> ostream& operator<< (ostream& os, const pair<T,U>& p) { os << '(' << p.first << ' ' << p.second << ')'; return os; }

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n)  FOR(i,0,n)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#ifdef LOCAL111
	#define DEBUG(x) cout<<#x<<": "<<(x)<<endl
	template<typename T> void dpite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
#else
	#define DEBUG(x) true
	template<typename T> void dpite(T a, T b){ return; }
#endif
#define F first
#define S second
#define SNP string::npos
#define WRC(hoge) cout << "Case #" << (hoge)+1 << ": "
template<typename T> void pite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}

template<typename T>
vector<T> make_v(size_t a){return vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value!=0>::type
fill_v(U &u,const V... v){u=U(v...);}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value==0>::type
fill_v(U &u,const V... v){
  for(auto &e:u) fill_v<T>(e,v...);
}

const array<int, 4> dx = {0, 1, 0, -1};
const array<int, 4> dy = {1, 0, -1, 0};


typedef long long int LL;
typedef unsigned long long ULL;
typedef pair<int,int> P;

void ios_init(){
	//cout.setf(ios::fixed);
	//cout.precision(12);
#ifdef LOCAL111
	return;
#endif
	ios::sync_with_stdio(false); cin.tie(0);
}

// 検証しなおしたい

//library
typedef long long cost_t;

class Edge {
public:
	int to = -1;
	cost_t cost = -1;
	
	Edge(){
	}

	Edge(int x,cost_t y){
		to = x;
		cost = y;
	}

	// Edge(int x, int y, cost_t z){
	// 	// from = x;
	// 	to = y;
	// 	cost = z;
	// }

	// Edge(int x, int y, int r, cost_t z){
	// 	// from = x;
	// 	to = y;
	// 	cost = z;
	// 	// rev = r;
	// }

	bool operator< (const Edge& x) const {
		if(cost != x.cost) return cost < x.cost;
		return to < x.to;
	}

	bool operator> (const Edge& x) const {
		if(cost != x.cost) return cost > x.cost;
		return to > x.to;
	}
};

class Graph {
private:
	typedef vector<Edge> Edges;
	//const long long int INF = (long long)1e18;
	vector<Edges> v; 
	int n;
public:
	Graph(int x){
		n = x;
		v = vector<Edges>(x);
	}

	Graph(){}

	Edges& operator[](int x){
		return v[x];
	}

	const Edges& operator[](int x) const {
		return v[x];
	}

	int size() const {
		return n;
	}

	void add_edge(int from, const Edge &e){
		v[from].push_back(e);
	}

	void add_edge(int from, int to, cost_t cost = -1){
		add_edge(from,Edge(to,cost));
	}

	// void add_edge(int from, int to, cost_t cost = -1, int rev = -1){
	// 	add_edge(from,Edge(from,to,rev,cost));
	// }

	void add_uedge(int from, int to, cost_t cost = -1){
		add_edge(from,to,cost);
		add_edge(to,from,cost);
	}
};

vector<cost_t> dijkstra(const Graph& v, int from)
{
	vector<cost_t> dist(v.size(),INF);
	priority_queue<Edge,vector<Edge>,greater<Edge>> que;
	que.push(Edge(from,0));
	while(!que.empty()){
		Edge e = que.top();
		que.pop();
		if(dist[e.to] == INF){
			dist[e.to] = e.cost;
			for(auto to : v[e.to]){
				if(dist[to.to] <= e.cost+to.cost) continue;
				que.push(Edge(to.to, e.cost+to.cost));
			}
		}
	}
	return dist;
}



class GraphVizualizer{
	int cnt;
public:
	GraphVizualizer(){
		cnt = 0;
	}

	void operator() (const Graph &g, bool with_dir = true, bool with_cap = false){
		ofstream ofs("./out.dot");
		if(!with_dir) ofs << "graph graph_name {" << endl;
		else ofs << "digraph graph_name {" << endl;
		for(int i = 0; i < (int)g.size(); ++i) {
			ofs << "    " << i << "[];" << endl;
		}
		for(int i = 0; i < (int)g.size(); i++){
			if (!g[i].size())
				continue;
			for(const auto &e : g[i]){
				if(!with_dir){
					if(e.to > i) ofs << "    " << i << " -- " << e.to ; 
				}else ofs << "    " << i << " -> " << e.to ; 
				
				if (with_cap) {
					ofs << " [ label = \"" << (e.cost  == INF ? "inf" : to_string(e.cost)) << "\"];"; 
				}
				if(with_dir or e.to > i)ofs << endl;
			}
		}
		ofs << "}" << endl;
		ofs.close();
		string cmd = "dot -T png out.dot > ";
		cmd = cmd+"sample"+to_string(cnt)+".png";
		system(cmd.c_str());	
		cnt++;
	}
};


int main()
{
	ios_init();
	int n, m;
	// GraphVizualizer viz; 
	while(cin >> n >> m) {
		
		using T = pair<LL, int>;
		
		vector<vector<T>> imos(n + 1);
		vector<vector<T>> imosr(n + 1);
		REP(i, m) {
			int l, r, c;
			cin >> l >> r >> c;
			l--;
			imos[l].emplace_back(c, i);
			imosr[r].emplace_back(c, i);
		}
		set<T> s;
		Graph g(n + m);
		REP(i, n + 1) {
			DEBUG(i);
			for(auto&& e : imos[i]) {
				s.insert(e);
				g.add_uedge(i, n + e.S, e.F);
			}
			for(auto&& e : imosr[i]) {
				g.add_uedge(i - 1, n + e.S, e.F);
				s.erase(e);
			}
			if(!s.empty()) {
				auto mi = *s.begin();
				DEBUG(mi);
				g.add_uedge(i, n + mi.S, mi.F);
				if(SZ(s) > 1) {
					auto mi = *(++s.begin());
					DEBUG(mi);
				
					g.add_uedge(i, n + mi.S, mi.F);
				}
			}
		}
		// REP(i, n) {
		// 	DEBUG(i);
		// 	for(auto&& e : g[i]) {
		// 		DEBUG(e.to); 
		// 		DEBUG(e.cost);
		// 	}
		// }
		// viz(g);
		LL ans = dijkstra(g, 0)[n - 1];
		if(ans >= INF) {
			cout << -1 << endl;
		} else {
			cout << ans / 2 << endl;

		}
	}
	return 0;
}
