#ifdef LOCAL
    #define _GLIBCXX_DEBUG
    #define __clock__
#else
    #pragma GCC optimize("Ofast")
    // #define NDEBUG
#endif
#define __precision__ 10
#define iostream_untie true
#define debug_stream std::cerr

#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <complex>
#include <cstring>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>

#define all(v) std::begin(v), std::end(v)
#define rall(v) std::rbegin(v), std::rend(v)
#define odd(n) ((n) & 1)
#define even(n) (not __odd(n))
#define __popcount(n) __builtin_popcountll(n)
#define __clz32(n) __builtin_clz(n)
#define __clz64(n) __builtin_clzll(n)
#define __ctz32(n) __builtin_ctz(n)
#define __ctz64(n) __builtin_ctzll(n)

using i32 = int_least32_t; using i64 = int_least64_t; using u32 = uint_least32_t; using u64 = uint_least64_t;
using pii = std::pair<i32, i32>; using pll = std::pair<i64, i64>;
template <class T> using heap = std::priority_queue<T>;
template <class T> using rheap = std::priority_queue<T, std::vector<T>, std::greater<T>>;
template <class K, class T> using hashmap = std::unordered_map<K, T>;
template <class T> using hashset = std::unordered_set<T>;

namespace execution
{
    using namespace std::chrono;
    system_clock::time_point start_time, end_time;
    long long get_elapsed_time() { end_time = system_clock::now(); return duration_cast<milliseconds>(end_time - start_time).count(); }
    void print_elapsed_time() { std::cerr << "\n----- Exec time : " << get_elapsed_time() << " ms -----\n\n"; }
    struct setupper
    {
        setupper()
        {
            if(iostream_untie) std::ios::sync_with_stdio(false), std::cin.tie(nullptr);
            std::cout << std::fixed << std::setprecision(__precision__);
    #ifdef stderr_path
            if(freopen(stderr_path, "a", stderr))
            {
                std::cerr << std::fixed << std::setprecision(__precision__);
            }
    #endif
    #ifdef stdout_path
            if(not freopen(stdout_path, "w", stdout))
            {
                freopen("CON", "w", stdout);
                std::cerr << "Failed to open the stdout file\n\n";
            }
            std::cout << "";
    #endif
    #ifdef stdin_path
            if(not freopen(stdin_path, "r", stdin))
            {
                freopen("CON", "r", stdin);
                std::cerr << "Failed to open the stdin file\n\n";
            }
    #endif
    #ifdef LOCAL
            std::cerr << "----- stderr at LOCAL -----\n\n";
            atexit(print_elapsed_time);
    #else
            fclose(stderr);
    #endif
    #ifdef __clock__
            start_time = system_clock::now();
    #endif
        }
    } __setupper;
    class myclock_t
    {
        system_clock::time_point built_pt, last_pt; int built_ln, last_ln;
        std::string built_func, last_func; bool is_built;
    public:
        myclock_t() : is_built(false) {}
        void build(int crt_ln, const std::string &crt_func)
        {
            is_built = true, last_pt = built_pt = system_clock::now(),  last_ln = built_ln = crt_ln, last_func = built_func = crt_func;
        }
        void set(int crt_ln, const std::string &crt_func)
        {
            if(is_built) last_pt = system_clock::now(), last_ln = crt_ln, last_func = crt_func;
            else debug_stream << "[ " << crt_ln << " : " << crt_func << " ] " << "myclock_t::set failed (yet to be built!)\n";
        }
        void get(int crt_ln, const std::string &crt_func)
        {
            if(is_built)
            {
                system_clock::time_point crt_pt(system_clock::now());
                long long diff = duration_cast<milliseconds>(crt_pt - last_pt).count();
                debug_stream << diff << " ms elapsed from" << " [ " << last_ln << " : " << last_func << " ]";
                if(last_ln == built_ln) debug_stream << " (when built)";
                debug_stream << " to" << " [ " << crt_ln << " : " << crt_func << " ]" << "\n";
                last_pt = built_pt, last_ln = built_ln, last_func = built_func;
            }
            else
            {
                debug_stream << "[ " << crt_ln << " : " << crt_func << " ] " << "myclock_t::get failed (yet to be built!)\n";
            }
        }
    };
} // namespace execution

#ifdef __clock__
    execution::myclock_t __myclock;
    #define build_clock() __myclock.build(__LINE__, __func__)
    #define set_clock() __myclock.set(__LINE__, __func__)
    #define get_clock() __myclock.get(__LINE__, __func__)
#else
    #define build_clock() ((void)0)
    #define set_clock() ((void)0)
    #define get_clock() ((void)0)
#endif

namespace std
{
    template <class P> void rsort(P __first, P __last) { sort(__first, __last, greater<>()); }
    template <class T> size_t hash_combine(size_t seed, T const &key) { return seed ^ (hash<T>()(key) + 0x9e3779b9 + (seed << 6) + (seed >> 2)); }
    template <class T, class U> struct hash<pair<T, U>> { size_t operator()(pair<T, U> const &pr) const { return hash_combine(hash_combine(0, pr.first), pr.second); } };
    template <class tuple_t, size_t index = tuple_size<tuple_t>::value - 1> struct tuple_hash_calc { static size_t apply(size_t seed, tuple_t const &t) { return hash_combine(tuple_hash_calc<tuple_t, index - 1>::apply(seed, t), get<index>(t)); } };
    template <class tuple_t> struct tuple_hash_calc<tuple_t, 0> { static size_t apply(size_t seed, tuple_t const &t) { return hash_combine(seed, get<0>(t)); } };
    template <class... T> struct hash<tuple<T...>> { size_t operator()(tuple<T...> const &t) const { return tuple_hash_calc<tuple<T...>>::apply(0, t); } };
    template <class T, class U> istream &operator>>(std::istream &s, pair<T, U> &p) { return s >> p.first >> p.second; }
    template <class T, class U> ostream &operator<<(std::ostream &s, const pair<T, U> &p) { return s << p.first << " " << p.second; }
    template <class T> istream &operator>>(istream &s, vector<T> &v) { for(T &e : v) s >> e; return s; }
    template <class T> ostream &operator<<(ostream &s, const vector<T> &v)  { bool is_front = true; for(const T &e : v) { if(not is_front) s << ' '; else is_front = false; s << e; } return s; }
    template <class tuple_t, size_t index> struct tupleos { static ostream &apply(ostream &s, const tuple_t &t) { tupleos<tuple_t, index - 1>::apply(s, t); return s << " " << get<index>(t); } };
    template <class tuple_t> struct tupleos<tuple_t, 0> { static ostream &apply(ostream &s, const tuple_t &t) { return s << get<0>(t); } };
    template <class... T> ostream &operator<<(ostream &s, const tuple<T...> &t)  { return tupleos<tuple<T...>, tuple_size<tuple<T...>>::value - 1>::apply(s, t); }
    template <> ostream &operator<<(ostream &s, const tuple<> &t) { return s; }
    string revstr(string str) { reverse(str.begin(), str.end()); return str; }
} // namespace std

#ifdef LOCAL
    #define dump(...)                                                              \
        debug_stream << "[ " << __LINE__ << " : " << __FUNCTION__ << " ]\n",       \
            dump_func(#__VA_ARGS__, __VA_ARGS__)
    template <class T> void dump_func(const char *ptr, const T &x)
    {
        debug_stream << '\t';
        for(char c = *ptr; c != '\0'; c = *++ptr) if(c != ' ') debug_stream << c;
        debug_stream << " : " << x << '\n';
    }
    template <class T, class... rest_t> void dump_func(const char *ptr, const T &x, rest_t... rest)
    {
        debug_stream << '\t';
        for(char c = *ptr; c != ','; c = *++ptr) if(c != ' ') debug_stream << c;
        debug_stream << " : " << x << ",\n"; dump_func(++ptr, rest...);
    }
#else
    #define dump(...) ((void)0)
#endif
template <class P> void read_range(P __first, P __second) { for(P i = __first; i != __second; ++i) std::cin >> *i; }
template <class P> void write_range(P __first, P __second) { for(P i = __first; i != __second; std::cout << (++i == __second ? '\n' : ' ')) std::cout << *i; }

// substitue y for x if x > y.
template <class T> inline bool sbmin(T &x, const T &y) { return x > y ? x = y, true : false; }
// substitue y for x if x < y.
template <class T> inline bool sbmax(T &x, const T &y) { return x < y ? x = y, true : false; }
// binary search.
i64 bin(const std::function<bool(i64)> &pred, i64 ok, i64 ng)
{
    while(std::abs(ok - ng) > 1) { i64 mid = (ok + ng) / 2; (pred(mid) ? ok : ng) = mid; }
    return ok;
}
double bin(const std::function<bool(double)> &pred, double ok, double ng, const double eps)
{
    while(std::abs(ok - ng) > eps) { double mid = (ok + ng) / 2; (pred(mid) ? ok : ng) = mid; }
    return ok;
}
// be careful that val is type-sensitive.
template <class T, class A, size_t N> void init(A (&array)[N], const T &val) { std::fill((T *)array, (T *)(array + N), val); }
// reset all bits.
template <class A> void reset(A &array) { memset(array, 0, sizeof(array)); }


/* The main code follows. */

using namespace std;

int main()
{
    void __solve();
    u32 t = 1;
#ifdef LOCAL
    t = 1;
#endif
    // t = -1;
    // cin >> t;
    while(t--)
    {
        __solve();
    }
}


#ifndef LAZY_SEGMENT_TREE_HPP
#define LAZY_SEGMENT_TREE_HPP

template <class Monoid, class Action>
class lazy_segment_tree
{
    using value_type = typename Monoid::value_type;
    using operand_type = typename Action::value_type;
    Monoid *const monoid_ptr, &monoid;
    Action *const action_ptr, &action;
    const size_t orig_n, height, ext_n;
    std::vector<value_type> data;
    std::vector<operand_type> lazy;

    void recalc(size_t node) { data[node] = monoid(data[node << 1], data[node << 1 | 1]); }

    void apply(size_t index, const operand_type &operand)
    {
        action.act(data[index], operand);
        if(index < ext_n) action(lazy[index], operand);
    }

    void push(size_t index)
    {
        apply(index << 1, lazy[index]);
        apply(index << 1 | 1, lazy[index]);
        lazy[index] = action.identity();
    }

    void left_bound(size_t index, const std::function<bool(const value_type &)> &pred,
                    size_t node, size_t begin, size_t end, value_type &now, size_t &res)
    {
        if(index <= begin || end < res) return;
        if(end <= index)
        {
            const value_type nxt = monoid(data[node], now);
            if(pred(nxt))
            {
                res = begin, now = nxt;
                return;
            }
        }
        if(end - begin > 1)
        {
            push(node);
            // search from right child
            left_bound(index, pred, node << 1 | 1, (begin + end) >> 1, end, now, res);
            left_bound(index, pred, node << 1, begin, (begin + end) >> 1, now, res);
            lazy[node] = action.identity();
        }
    }

    void right_bound(size_t index, const std::function<bool(const value_type &)> &pred,
                    size_t node, size_t begin, size_t end, value_type &now, size_t &res)
    {
        if(index >= end || begin > res) return;
        if(begin >= index)
        {
            const value_type nxt = monoid(now, data[node]);
            if(pred(nxt))
            {
                res = end, now = nxt;
                return;
            }
        }
        if(end - begin > 1)
        {
            push(node);
            // search from left child
            right_bound(index, pred, node << 1, begin, (begin + end) >> 1, now, res);
            right_bound(index, pred, node << 1 | 1, (begin + end) >> 1, end, now, res);
            lazy[node] = action.identity();
        }
    }

  public:
    explicit lazy_segment_tree(size_t n) : monoid_ptr{new Monoid}, monoid{*monoid_ptr}, action_ptr{new Action}, action{*action_ptr},
                                            orig_n{n}, height(n > 1 ? 32 - __builtin_clz(n - 1) : 0), ext_n(1 << height),
                                            data(ext_n << 1, monoid.identity()), lazy(ext_n, action.identity()) {}
    lazy_segment_tree(size_t n, Monoid &_monoid) : monoid_ptr{}, monoid{_monoid}, action_ptr{new Action}, action{*action_ptr},
                                                    orig_n{n}, height(n > 1 ? 32 - __builtin_clz(n - 1) : 0), ext_n(1 << height),
                                                    data(ext_n << 1, monoid.identity()), lazy(ext_n, action.identity()) {}
    lazy_segment_tree(size_t n, Action &_actor) : monoid_ptr{new Monoid}, monoid{*monoid_ptr}, action_ptr{}, action{_actor},
                                                    orig_n{n}, height(n > 1 ? 32 - __builtin_clz(n - 1) : 0), ext_n(1 << height),
                                                    data(ext_n << 1, monoid.identity()), lazy(ext_n, action.identity()) {}
    lazy_segment_tree(size_t n, Monoid &_monoid, Action &_actor) : monoid_ptr{}, monoid{_monoid}, action_ptr{}, action{_actor},
                                                                    orig_n{n}, height(n > 1 ? 32 - __builtin_clz(n - 1) : 0), ext_n(1 << height),
                                                                    data(ext_n << 1, monoid.identity()), lazy(ext_n, action.identity()) {}
    ~lazy_segment_tree() { delete monoid_ptr; delete action_ptr; }

    // copy of the element at position i.
    value_type operator[](size_t i) { return fold(i, i + 1); }

    void build(value_type *__first, value_type *__last)
    {
        assert((size_t)std::distance(__first, __last) <= ext_n);
        std::copy(__first, __last, data.begin() + ext_n);
        for(size_t i = ext_n - 1; i; --i) recalc(i);
    }

    template <class iterator>
    void build(iterator __first, iterator __last)
    {
        static_assert(std::is_same<typename std::iterator_traits<iterator>::value_type, value_type>::value, "iterator's value_type should be equal to Monoid's");
        assert((size_t)std::distance(__first, __last) <= ext_n);
        std::copy(__first, __last, data.begin() + ext_n);
        for(size_t i = ext_n - 1; i; --i) recalc(i);
    }

    void init(const value_type &x)
    {
        for(size_t i = 0; i != ext_n; ++i) data[i | ext_n] = x;
        for(size_t i = ext_n - 1; i; --i) recalc(i);
    }

    void update(size_t index, const operand_type &operand) { update(index, index + 1, operand); }

    void update(size_t begin, size_t end, const operand_type &operand)
    {
        assert(0 <= begin && end <= orig_n);
        begin += ext_n, end += ext_n - 1;
        for(size_t i = height; i; --i) push(begin >> i), push(end >> i);
        for(size_t l = begin, r = end + 1; end; l >>= 1, r >>= 1)
        {
            if(l < r)
            {
                if(l & 1) apply(l++, operand);
                if(r & 1) apply(--r, operand);
            }
            if(begin >>= 1, end >>= 1)
            {
                recalc(begin), recalc(end);
            }
        }
    }

    value_type fold(size_t begin, size_t end)
    {
        assert(0 <= begin && end <= orig_n);
        begin += ext_n, end += ext_n - 1;
        value_type left_val{monoid.identity()}, right_val{monoid.identity()};
        for(size_t l = begin, r = end + 1; end; l >>= 1, r >>= 1)
        {
            if(l < r)
            {
                if(l & 1) left_val = monoid(left_val, data[l++]);
                if(r & 1) right_val = monoid(data[--r], right_val);
            }
            if(begin >>= 1, end >>= 1)
            {
                action.act(left_val, lazy[begin]);
                action.act(right_val, lazy[end]);
            }
        }
        return monoid(left_val, right_val);
    }

    // minimum l where range [l, index) meets the condition.
    size_t left_bound(size_t index, const std::function<bool(const value_type &)> &pred)
    {
        assert(index <= orig_n);
        size_t res = index;
        value_type now = monoid.identity();
        left_bound(index, pred, 1, 0, ext_n, now, res);
        return res;
    }

    // maximum r where range [index, r) meets the condition.
    size_t right_bound(size_t index, const std::function<bool(const value_type &)> &pred)
    {
        assert(index < orig_n);
        size_t res = index;
        value_type now = monoid.identity();
        right_bound(index, pred, 1, 0, ext_n, now, res);
        return res < orig_n ? res : orig_n;
    }
}; //class lazy_segment_tree

#endif


void __solve()
{
    int n; cin>>n;
    int m; cin>>m;
    vector<vector<pii>> fro(n),to(n);
    for(i32 i=0; i<m; ++i)
    {
        int l,r,c; cin>>l>>r>>c;
        l--; r--;
        fro[l].emplace_back(r,c);
        to[r].emplace_back(l,c);
    }

    const i64 inf=1e18;

    struct mono
    {
        using value_type=i64;
        i64 operator()(i64 x,i64 y)
        {
            return min(x,y);
        }
        i64 identity()
        {
            return inf;
        }
    };

    struct action
    {
        using value_type=i64;
        void operator()(i64 &x,i64 y)
        {
            x=min(x,y);
        }
        void act(i64 &x,i64 y)
        {
            x=min(x,y);
        }
        i64 identity()
        {
            return inf;
        }
    };

    lazy_segment_tree<mono,action> seg(n);

    seg.update(0,0);

    for(i32 i=0; i<n; ++i)
    {
        for(auto itv : to[i])
        {
            int l,c; tie(l,c)=itv;
            seg.update(i,seg.fold(l,i)+c);
        }
        for(auto itv : fro[i])
        {
            int r,c; tie(r,c)=itv;
            seg.update(i,r+1,seg[i]+c);
        }
    }

    if(seg[n-1]!=inf)
    {
        cout << seg[n-1] << "\n";
    }
    else
    {
        cout << -1 << "\n";
    }

}


Measured memory_bytes: 14080