// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>
#include <fstream>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest& ...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest& ...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest& ...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)std::cin>>w;
// output
template<typename T>void MACRO_OUT(const T t) { std::cout << t; }
template<typename First, typename...Rest>void MACRO_OUT(const First first, const Rest...rest) { std::cout << first << " "; MACRO_OUT(rest...); }
#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define FOUT(n, dist) std::cout<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) std::cout<<std::setw(n)<<std::setfill(c)<<(dist);
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t: " << (dist) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t: ";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
#define SHOWSTACK(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.top() << " ";tmp.pop();}std::cerr << "\n";}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define REP(w, n) for(int w=0;w<int(n);++w)
#define RREP(w, n) for(int w=int(n)-1;w>=0;--w)
#define IN(a, x, b) (a<=x && x<b)
template<class T> inline T CHMAX(T & a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                    // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-10;
constexpr int MOD = 998244353;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T> & a, const T & val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>> & a, const T & val) { for (auto& b : a) FILL(b, val); }
// ------------>8------------------------------------->8------------

// ------------>8------------ SegmentTree ------------>8------------

namespace Monoid {
	template <class T> class sum {
	public:
		using value_type = T;
		inline static T operation(const T& a, const T& b) { return a + b; }
		inline static T identity() { return T(0); }
	};
	template <class T> class max {
	public:
		using value_type = T;
		inline static T operation(const T& a, const T& b) { return (a < b) ? b : a; }
		inline static T identity() { return std::numeric_limits<T>::lowest(); }
	};
	template <class T> class min {
	public:
		using value_type = T;
		inline static T operation(const T& a, const T& b) { return (a < b) ? a : b; }
		inline static T identity() { return std::numeric_limits<T>::max(); }
	};

	class MONOID {
	public:
		struct S { int x; };
		using value_type = S;
		inline static S operation(const S& l, const S& r) { return S{ l.x + r.x }; }
		inline static S identity() { return S{ 0 }; }
	};
}
template <class Monoid>
class SegmentTree {
private:
	using T = typename Monoid::value_type;
	int ARY_SIZE;
	std::vector<T> ary;

	void init(int n) {
		while (ARY_SIZE < n) ARY_SIZE <<= 1;
		ary.resize(ARY_SIZE << 1, Monoid::identity());
	}
public:
	SegmentTree(int n) : ARY_SIZE(1) { init(n); }
	SegmentTree(std::vector<T>& a) : ARY_SIZE(1) {
		init(a.size());
		std::copy(a.begin(), a.end(), ary.begin() + ARY_SIZE);
		for (int i = ARY_SIZE - 1; i >= 1; --i) {
			ary[i] = Monoid::operation(ary[i << 1], ary[(i << 1) + 1]);
		}
	}

	// -- a[i] = val;
	inline void update(int i, T val) {
		i += ARY_SIZE;
		ary[i] = val;
		while (i > 1) {
			i >>= 1;
			ary[i] = Monoid::operation(ary[i << 1], ary[(i << 1) + 1]);
		}
	}
	// -- a[i] += val;
	inline void add(int i, T val) {
		update(i, ary[i + ARY_SIZE] + val);
	}
	// -- [l, r)
	inline T query(int l, int r) {
		if (l >= r) return Monoid::identity();
		T vl = Monoid::identity(), vr = Monoid::identity();
		for (l += ARY_SIZE, r += ARY_SIZE; l != r; l >>= 1, r >>= 1) {
			if (l & 1) vl = Monoid::operation(vl, ary[l++]);
			if (r & 1) vr = Monoid::operation(ary[--r], vr);
		}
		return Monoid::operation(vl, vr);
	}
	T operator[](int i) { return ary[i + ARY_SIZE]; }

	void debugShow() {
		for (int i = ARY_SIZE; i < ARY_SIZE << 1; ++i) std::cerr << ary[i] << " ";
		std::cerr << "\n";
	}
};
// ------------8<------------ SegmentTree ------------8<------------

signed main() {
	INIT;

	VAR(int, n, m);
	VEC_ROW(int, m, l, r, c);

	VV<PAIR> RL(n);
	REP(i, m) {
		--l[i]; --r[i];
		RL[r[i]].emplace_back(l[i], c[i]);
	}

	SegmentTree<Monoid::min<int>> st(n);
	st.update(0, 0);

	FOR(i, 1, n) {
		int res = INFLL;
		for (auto& p : RL[i]) {
			CHMIN(res, st.query(p.first, i) + p.second);
		}
		st.update(i, res);
	}

	int ans = st.query(n - 1, n);
	if (ans >= INFLL) ans = -1;
	OUT(ans)BR;

	return 0;
}

Measured memory_bytes: 9984