#include <iostream>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <map>
#include <set>
#include <algorithm>
#include <queue>
#include <stack>
#include <functional>

using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef pair<ll,ll> P;
typedef vector<P> vpl;
#define rep(i,n) for(ll i=0; i<(n); i++)
#define REP(i,a,b) for(ll i=(a); i<(b); i++)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const int inf = 1<<30;
const ll linf = 1LL<<62;
const int mod = 1e9+7;
const int MAX = 510000;
const int V = 100005;
ll dy[8] = {1,0,-1,0,1,-1,1,-1};
ll dx[8] = {0,1,0,-1,1,-1,-1,1};
const double pi = acos(-1);
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}
template<typename T1,typename T2> inline void print2(T1 a, T2 b){cout << a << " " << b << endl;}
#define INT_MAX 1e9+7

template< typename Monoid, typename OperatorMonoid = Monoid >
struct LazySegmentTree {
    using F = function< Monoid(Monoid, Monoid) >;
    using G = function< Monoid(Monoid, OperatorMonoid) >;
    using H = function< OperatorMonoid(OperatorMonoid, OperatorMonoid) >;

    int sz, height;
    vector< Monoid > data;
    vector< OperatorMonoid > lazy;
    const F f;
    const G g;
    const H h;
    const Monoid M1;
    const OperatorMonoid OM0;


    LazySegmentTree(int n, const F f, const G g, const H h,
                    const Monoid &M1, const OperatorMonoid OM0)
            : f(f), g(g), h(h), M1(M1), OM0(OM0) {
        sz = 1;
        height = 0;
        while(sz < n) sz <<= 1, height++;
        data.assign(2 * sz, M1);
        lazy.assign(2 * sz, OM0);
    }

    void set(int k, const Monoid &x) {
        data[k + sz] = x;
    }

    void build() {
        for(int k = sz - 1; k > 0; k--) {
            data[k] = f(data[2 * k + 0], data[2 * k + 1]);
        }
    }

    inline void propagate(int k) {
        if(lazy[k] != OM0) {
            lazy[2 * k + 0] = h(lazy[2 * k + 0], lazy[k]);
            lazy[2 * k + 1] = h(lazy[2 * k + 1], lazy[k]);
            data[k] = reflect(k);
            lazy[k] = OM0;
        }
    }

    inline Monoid reflect(int k) {
        return lazy[k] == OM0 ? data[k] : g(data[k], lazy[k]);
    }

    inline void recalc(int k) {
        while(k >>= 1) data[k] = f(reflect(2 * k + 0), reflect(2 * k + 1));
    }

    inline void thrust(int k) {
        for(int i = height; i > 0; i--) propagate(k >> i);
    }

    void update(int a, int b, const OperatorMonoid &x) {
        thrust(a += sz);
        thrust(b += sz - 1);
        for(int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
            if(l & 1) lazy[l] = h(lazy[l], x), ++l;
            if(r & 1) --r, lazy[r] = h(lazy[r], x);
        }
        recalc(a);
        recalc(b);
    }

    Monoid query(int a, int b) {
        thrust(a += sz);
        thrust(b += sz - 1);
        Monoid L = M1, R = M1;
        for(int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
            if(l & 1) L = f(L, reflect(l++));
            if(r & 1) R = f(reflect(--r), R);
        }
        return f(L, R);
    }

    Monoid operator[](const int &k) {
        return query(k, k + 1);
    }

    template< typename C >
    int find_subtree(int a, const C &check, Monoid &M, bool type) {
        while(a < sz) {
            propagate(a);
            Monoid nxt = type ? f(reflect(2 * a + type), M) : f(M, reflect(2 * a + type));
            if(check(nxt)) a = 2 * a + type;
            else M = nxt, a = 2 * a + 1 - type;
        }
        return a - sz;
    }

    template< typename C >
    int find_first(int a, const C &check) {
        Monoid L = M1;
        if(a <= 0) {
            if(check(f(L, reflect(1)))) return find_subtree(1, check, L, false);
            return -1;
        }
        thrust(a + sz);
        int b = sz;
        for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
            if(a & 1) {
                Monoid nxt = f(L, reflect(a));
                if(check(nxt)) return find_subtree(a, check, L, false);
                L = nxt;
                ++a;
            }
        }
        return -1;
    }


    template< typename C >
    int find_last(int b, const C &check) {
        Monoid R = M1;
        if(b >= sz) {
            if(check(f(reflect(1), R))) return find_subtree(1, check, R, true);
            return -1;
        }
        thrust(b + sz - 1);
        int a = sz;
        for(b += sz; a < b; a >>= 1, b >>= 1) {
            if(b & 1) {
                Monoid nxt = f(reflect(--b), R);
                if(check(nxt)) return find_subtree(b, check, R, true);
                R = nxt;
            }
        }
        return -1;
    }
};


int main(){
    ll n,m; cin >> n >> m;
    vector<tuple<ll,ll,ll>> edges(m);
    rep(i,m){
        ll l,r,c; cin >> l >> r >> c;
        edges[i] = tie(l,r,c);
    }
    sort(all(edges));
    auto f = [](ll a, ll b){return min(a,b);};
    auto g = [](ll a, ll b){return min(a,b);};
    auto h = [](ll a, ll b){return min(a,b);};
    LazySegmentTree<ll> seg(n,f,g,h,linf,linf);
    seg.update(0,1,0);
    rep(i,m){
        ll l,r,c;
        tie(l,r,c) = edges[i];
        l--; r--;
        ll t = seg.query(l,l+1);
        seg.update(l,r+1,c+t);
        /*cout << t << endl;
        rep(i,n-2) cout << seg.query(i,i+1) << " ";
        cout << endl;*/
    }
    ll ans = seg.query(n-1,n);
    if(ans == linf) puts("-1");
    else cout << ans << endl;
}


Measured memory_bytes: 6656