#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout<<(ans)<<endl
#define STI(s) atoi(s.c_str())
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
#define itn int
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);};};
const int MOD = 1000000007;
const int INF = INT_MAX;
const ll LLINF = 1LL<<62;

class DIJKSTRA {
public:
    ll V;

    struct edge {
        ll to;
        ll cost;
    };

    typedef pair<ll, ll> PI; //firstは最短距離、secondは頂点の番号
    vector<vector<edge> >G;
    vector<ll> d; //これ答え。d[i]:=V[i]までの最短距離
    vector<ll> prev; //経路復元

    DIJKSTRA(ll size) {
        V = size;
        G = vector<vector<edge> >(V);
        prev = vector<ll> (V, -1);
    }

    void add(ll from, ll to, ll cost) {
        edge e = {to, cost};
        G[from].push_back(e);
    }

    void dijkstra(ll s) {
        //greater<P>を指定することでfirstが小さい順に取り出せるようにする
        priority_queue<PI, vector<PI>, greater<PI> > que;
        d = vector<ll> (V, LLINF);
        d[s] = 0;
        que.push(PI(0, s));

        while(!que.empty()) {
            PI p = que.top();
            que.pop();
            ll v = p.second;
            if(d[v] < p.first) continue;
            for(ll i = 0; i < G[v].size(); i++) {
                edge e = G[v][i];
                if(d[e.to] > d[v] + e.cost) {
                    d[e.to] = d[v] + e.cost;
                    prev[e.to] = v;
                    que.push(PI(d[e.to], e.to));
                }
            }
        }
    }
    vector<ll> get_path(ll t) {
        vector<ll> path;
        for(;t != -1; t = prev[t]) {
            //tがsになるまでprev[t]をたどっていく
            path.push_back(t);
        }
        //このままだとt->sの順になっているので逆順にする
        reverse(path.begin(),path.end());
        return path;
    }
    void show() {
        for(ll i = 0; i < d.size()-1; i ++) {
            cout << d[i] << " ";
        }
        cout << d[d.size()-1] << endl;
    }
};

int main(void) {

    ll n,m;
    cin>>n>>m;
    DIJKSTRA dj(n);

    for(ll i=0;i<m;i++){
        ll l,r,c;
        cin>>l>>r>>c;
        dj.add(l-1,r-1,c);
    }
    for(ll i=0;i<n-1;i++){
        dj.add(i+1,i,0);
    }
    dj.dijkstra(0);
    cout<<((dj.d[n-1]==LLINF)?-1:dj.d[n-1])<<endl;


}


Measured memory_bytes: 10104