#include <bits/stdc++.h>
using namespace std;
#if __has_include(<boost/multiprecision/cpp_int.hpp>)
	#include <boost/multiprecision/cpp_int.hpp>
	#include <boost/multiprecision/cpp_dec_float.hpp>
	using bll = boost::multiprecision::cpp_int;
	using bdouble = boost::multiprecision::cpp_dec_float_100;
	using namespace boost::multiprecision;
#endif
#if __has_include(<boost/stacktrace.hpp>)
	#define BOOST_STACKTRACE_USE_ADDR2LINE
	#define BOOST_STACKTRACE_ADDR2LINE_LOCATION /usr/local/opt/binutils/bin/addr2line
	#define _GNU_SOURCE 1
	#include <boost/stacktrace.hpp>
#endif
#ifdef LOCAL_TEST
	namespace std {
		template<typename T> class dvector : public std::vector<T> {
		public:
			dvector() : std::vector<T>() {}
			explicit dvector(size_t n, const T& value = T()) : std::vector<T>(n, value) {}
			dvector(const std::vector<T>& v) : std::vector<T>(v) {}
			dvector(const std::initializer_list<T> il) : std::vector<T>(il) {}
			dvector(const std::string::iterator first, const std::string::iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::iterator first, const typename std::vector<T>::iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::reverse_iterator first, const typename std::vector<T>::reverse_iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::const_iterator first, const typename std::vector<T>::const_iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::const_reverse_iterator first, const typename std::vector<T>::const_reverse_iterator last) : std::vector<T>(first, last) {}
			T& operator[](size_t n) {
				try { return this->at(n); } catch (const std::exception& e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; return this->at(n); }
			}
			const T& operator[](size_t n) const {
				try { return this->at(n); } catch (const std::exception& e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; return this->at(n); }
			}
		};
	}
	class dbool {
	private:
		bool boolvalue;
	public:
		dbool() : boolvalue(false) {}
		dbool(bool b) : boolvalue(b) {}
		operator bool&() { return boolvalue; }
		operator const bool&() const { return boolvalue; }
	};
	#define vector dvector
	#define bool dbool
#endif
#ifdef LOCAL_DEV
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::vector<T>& v) {
		for (size_t i = 0; i < v.size(); ++i){ s << v[i]; if (i < v.size() - 1) s << "\t"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::vector<std::vector<T>>& vv) {
		s << "\\\n"; for (size_t i = 0; i < vv.size(); ++i){ s << vv[i] << "\n"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::deque<T>& v) {
		for (size_t i = 0; i < v.size(); ++i){ s << v[i]; if (i < v.size() - 1) s << "\t"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::set<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}"; return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::multiset<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}"; return s; }
	template <typename T, size_t N> std::ostream& operator<<(std::ostream& s, const std::array<T, N>& a) {
		s << "{ "; for (size_t i = 0; i < N; ++i){ s << a[i] << "\t"; } s << "}"; return s; }
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::map<T1, T2>& m) {
		s << "{\n"; for (auto itr = m.begin(); itr != m.end(); ++itr){ s << "\t" << (*itr).first << " : " << (*itr).second << "\n"; } s << "}"; return s; }
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::pair<T1, T2>& p) {
		return s << "(" << p.first << ", " << p.second << ")"; }
	class SIGFPE_exception : std::exception {};
	class SIGSEGV_exception : std::exception {};
	void catch_SIGFPE([[maybe_unused]] int e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; throw SIGFPE_exception(); }
	void catch_SIGSEGV([[maybe_unused]] int e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; throw SIGSEGV_exception(); }
	signed convertedmain();
	signed main() { signal(SIGFPE, catch_SIGFPE); signal(SIGSEGV, catch_SIGSEGV); return convertedmain(); }
	#define main() convertedmain()
	void debug_impl() { std::cerr << '\n'; }
	template<typename Head, typename... Tail> void debug_impl(Head head, Tail... tail) { std::cerr << " " << head << (sizeof...(tail) ? "," : ""); debug_impl(tail...); }
	#define debug(...) do { std::cerr << "(" << #__VA_ARGS__ << ") ="; debug_impl(__VA_ARGS__); } while (false)
#else
	#define debug(...) do {} while (false)
#endif
//#define int long long
using ll = long long;
//constexpr int INF = (ll)1e9 + 7;//INT_MAX=(1<<31)-1=2147483647
constexpr ll INF = (ll)1e18;//(1LL<<63)-1=9223372036854775807
constexpr ll MOD = (ll)1e9 + 7;
constexpr double EPS = 1e-9;
constexpr ll dx[4] = {1, 0, -1, 0};
constexpr ll dy[4] = {0, 1, 0, -1};
constexpr ll dx8[8] = {1, 0, -1, 0, 1, 1, -1, -1};
constexpr ll dy8[8] = {0, 1, 0, -1, 1, -1, 1, -1};
#define rep(i, n)   for(ll i=0, i##_length=(n); i< i##_length; ++i)
#define repeq(i, n) for(ll i=1, i##_length=(n); i<=i##_length; ++i)
#define rrep(i, n)   for(ll i=(n)-1; i>=0; --i)
#define rrepeq(i, n) for(ll i=(n)  ; i>=1; --i)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
void p() { std::cout << '\n'; }
template<typename Head, typename... Tail> void p(Head head, Tail... tail) { std::cout << head << (sizeof...(tail) ? " " : ""); p(tail...); }
template<typename T> inline void pv(std::vector<T>& v) { for(ll i=0, N=v.size(); i<N; i++) std::cout << v[i] << " \n"[i==N-1]; }
template<typename T> inline bool chmax(T& a, T b) { return a < b && (a = b, true); }
template<typename T> inline bool chmin(T& a, T b) { return a > b && (a = b, true); }
template<typename T> inline void uniq(std::vector<T>& v) { v.erase(std::unique(v.begin(), v.end()), v.end()); }

/*-----8<-----template-----8<-----*/

class Edge {
public:
	ll from,to,cost;
	Edge() {}
	Edge(ll from, ll to, ll cost) : from(from), to(to), cost(cost) {}
};
ostream& operator<<(ostream& s, const Edge& e) {
	s << "{ " << e.from << " -> " << e.to << ", " << e.cost << " }";
	return s;
}
//セグ木の形にして区間に辺を張る
class Graph {
public:
	ll N;
	ll sz;
	vector<vector<Edge>> g;
	Graph (ll N) : N(N) {
		sz = 1;
		while(sz < N) sz <<= 1;
		g.assign(sz*5-2, vector<Edge>());
		ll diff=sz;
		for(ll i=0;i<sz-1;i++){
			g[i*2+1+diff].emplace_back(i*2+1+diff, i+diff, 0);
			g[i*2+2+diff].emplace_back(i*2+2+diff, i+diff, 0);
		}
		for(ll i=0;i<sz;i++){
			g[i].emplace_back(0, i+sz*2-1, 0);
		}
		diff=sz*3-1;
		for(ll i=0;i<sz-1;i++){
			g[i+diff].emplace_back(i+diff, i*2+1+diff, 0);
			g[i+diff].emplace_back(i+diff, i*2+2+diff, 0);
		}
		for(ll i=0;i<sz;i++){
			g[i+sz*4-2].emplace_back(i+sz*4-2, i, 0);
		}
	};

	//[froml, fromr) -> [tol, tor) に 距離costの辺を張る
	void add_edge(ll froml, ll fromr, ll tol, ll tor, ll cost){
		g.emplace_back();
		g.emplace_back();
		ll from=g.size()-2;
		ll to=g.size()-1;
		g[from].emplace_back(from,to,cost);
		recursive(froml, fromr, 0, 0, sz, -1, from, sz);
		recursive(tol, tor, 0, 0, sz, to, -1, sz*3-1);
	}
	// 要求区間 [a, b) 中の要素の最小値を答える
	// k := 自分がいるノードのインデックス
	// 対象区間は [l, r) にあたる
	void recursive(ll a, ll b, ll k, ll l, ll r, ll from, ll to, ll diff) {
		// 要求区間と対象区間が交わらない -> 適当に返す
		if(r <= a || b <= l) return;

		// 要求区間が対象区間を完全に被覆 -> 対象区間を答えの計算に使う
		if(a <= l && r <= b) {
			ll v=k+diff;
			if(from!=-1){
				g[from].emplace_back(from, v, 0);
			}else{
				g[v].emplace_back(v, to, 0);
			}
			return;
		}

		// 要求区間が対象区間の一部を被覆 -> 子について探索を行う
		// 左側の子を vl ・ 右側の子を vr としている
		// 新しい対象区間は、現在の対象区間を半分に割ったもの
		recursive(a, b, 2*k+1, l, (l+r)/2, from, to, diff);
		recursive(a, b, 2*k+2, (l+r)/2, r, from, to, diff);
	}

	vector<ll> dijkstra(ll start) {
		vector<ll> dist; vector<ll> prev;
		ll gsize = g.size();
		dist.assign(gsize, INF); dist[start] = 0;
		prev.assign(gsize, -1);
	
		priority_queue<Edge, vector<Edge>, function<bool(Edge,Edge)>> que(
			[](const Edge &x, const Edge &y){
				return x.cost != y.cost ? (x.cost > y.cost) : 
					(x.from != y.from ? x.from < y.from : x.to < y.to);
			}
		);
		que.emplace(-1, start, 0);
		while (!que.empty()) {
			Edge e = que.top();
			que.pop();
			if (prev[e.to] != -1) continue;
			prev[e.to] = e.from;
			for(auto&& f : g[e.to]) {
				if (dist[f.to] > e.cost + f.cost) {
					dist[f.to] = e.cost + f.cost;
					que.emplace(f.from, f.to, e.cost+f.cost);
				}
			}
		}
		vector<ll> ans(N);
		for(ll i=0;i<N;i++)ans[i]=dist[i];
		return ans;
	}
};

/*-----8<-----library-----8<-----*/

void solve() {
	ll N,M;
	cin>>N>>M;

	Graph g(N);
	rep(i,M){
		ll a,b,c;
		cin>>a>>b>>c;
		a--;
		g.add_edge(a,b,a,b,c);
	}
	vector<ll> dist=g.dijkstra(0);
	debug(dist);
	
	ll ans=dist.back();
	if(ans==INF)ans=-1;
	p(ans);
}

signed main() {
	solve();
	return 0;
}


Measured memory_bytes: 157312