#include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os, const map<T,U>& mp){ os << "{"; for(auto& p : mp){ os << p << ","; } os << "}"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T>& st){ os<<"{"; for(T v:st) os<<v<<","; os <<"}"; return os; }
template<typename T,typename U> inline void chmax(T &x,U y){ if(y>x) x = y; }
template<typename T,typename U> inline void chmin(T &x,U y){ if(y<x) x = y; }
using ll = long long;
using ull = unsigned long long;
using pii = std::pair<int,int>;
using vi = std::vector<int>;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n - 1); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/


int main(){
    int n,m; cin >> n >> m;
    set<int> stl,str;
    vector<tuple<int,int,ll>> v;
    st.insert(0);
    st.insert(n-1);
    rep(i,m){
        int l,r; cin >> l >> r; l--;r--;
        ll c; cin >> c;
        stl.insert(l);
        str.insert(r);
        v.emplace_back(l,r,c);
    }

    vector<vector<pair<int,ll>>> G(n);
    for(auto& t : v){
        int l,r; ll c;
        tie(l,r,c) = t;
        auto si = str.upper_bound(l);
        auto se = str.upper_bound(r);
        for(auto iter = si; iter!=se ; iter++){
            G[l].emplace_back(*iter,c);
        }
    }

    /* rep(i,n){ */
    /*     cout << G[i] << endl; */
    /* } */

    vector<ll> dp(n,LINF);
    dp[0] = 0;
    for(int i=0;i<n-1;i++){
        for(auto& p : G[i]){
            int v = p.first;
            ll c = p.second;
            dp[v] = min(dp[v],dp[i]+c);
        }
    }
    if(dp[n-1]==LINF){
        cout << -1 << endl;
        return 0;
    }
    //cerr << dp << endl;
    cout << dp[n-1] << endl;
}
