//#define _GLIBCXX_DEBUG

#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (ll)(m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T1, typename T2>
bool chmin(T1 &a,T2 b){if(a>b){a=b;return true;}else return false;}
template<typename T1, typename T2>
bool chmax(T1 &a,T2 b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,-1,0,0,1,1,-1,-1};
vector<ll>dy={0,0,1,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2>&p){
  return os << p.first << " " << p.second;
}
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
struct edge {
  ll to,cost;
  edge(ll x,ll y):to(x),cost(y){};
};
struct RangeGraph{
  vector<vector<edge>>g;
  ll n,sz;
  ll cnt;
  RangeGraph(ll n,ll m):n(n){
    sz=1;
    cnt=0;
    while(sz<n)sz <<= 1;
    g.assign(4*sz+n+m,vector<edge>());
    for(ll i=1;i<sz;i++){
      g[i].EB(2*i,0);
      g[i].EB(2*i+1,0);
      g[2*i+2*sz].EB(i+2*sz,0);
      g[2*i+1+2*sz].EB(i+2*sz,0);
    }
    for(ll i=0;i<n;i++){
      g[i+sz].EB(i+4*sz,0);
      g[i+4*sz].EB(i+3*sz,0);
    }
  }
  void add_edge(ll l1,ll r1,ll l2,ll r2,ll w){//[l1,r1)->[l2,r2)
    auto v=range(l1,r1);
    ll via=4*sz+n+cnt;
    for(auto z:v)g[z+2*sz].EB(via,w);
    v=range(l2,r2);
    for(auto z:v)g[via].EB(z,0);
    cnt++;
  }
  vector<ll>range(ll l,ll r){
    vector<ll>ret;
    l+=sz,r+=sz;
    while(l!=r){
      if(l&1)ret.PB(l++);
      if(r&1)ret.PB(--r);
      l>>=1,r>>=1;
    }
    return ret;
  };
  ll add(){
    return 4*sz;
  }
};
ll MAX_V = 1e6 + 5;
vector<ll> dist(MAX_V);
void dijkstra(ll s,vector<vector<edge>>&g) {
    priority_queue<P, vector<P>, greater<P> > que;
    fill(dist.begin(),dist.begin()+g.size(), INF);
    dist[s] = 0;
    que.emplace(0, s);
    while (!que.empty()) {
        P p = que.top();
        que.pop();
        ll v = p.second;
        if (dist[v] < p.first) continue;
        for(auto e:g[v]) {
            if (dist[e.to] > dist[v] + e.cost) {
                dist[e.to] = dist[v] + e.cost;
                que.emplace(dist[e.to], e.to);
            }
        }
    }
}
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  ll n,m;cin>>n>>m;
  RangeGraph rg(n,m);
  rep(i,0,m){
    ll l,r,c;cin>>l>>r>>c;l--;
    rg.add_edge(l,r,l,r,c);
  }
  ll add=rg.add();
  dijkstra(add,rg.g);
  ans(dist[add+n-1]==INF,-1,dist[add+n-1]);
  return 0;
}

Measured memory_bytes: 112184