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

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define test cout<<"test"<<endl;
#define fi first
#define se second
#define MP make_pair
#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 = m - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T>
void chmin(T &a,T b){if(a>b)a=b;}
template<typename T>
void chmax(T &a,T b){if(a<b)a=b;}
void pmod(ll &a,ll b){a=(a+b)%MOD;}
void pmod(ll &a,ll b,ll c){a=(b+c)%MOD;}
void qmod(ll &a,ll b){a=(a*b)%MOD;}
void qmod(ll &a,ll b,ll c){a=(b*c)%MOD;}
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={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-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 T>
struct SegmentTree{
  using F = function<T(T,T)>;
  vector<T> data;
  ll n,lastlen = 1;
  F func = [](T a, T b){return a < b ? a : b;};
  T iden = LLONG_MAX; //identity element
  SegmentTree(vector<T> v){
    n = (ll)v.size();
    while(lastlen < n)lastlen *= 2;
    data.assign(lastlen*2-1,iden);
    for(ll i=0;i<n;i++)data[i+lastlen-1] = v[i];
    for(ll i=lastlen-2;i>=0;i--){
      data[i] = func(data[2*i+1], data[2*i+2]);
    }
  }
  void update(ll point, T x){
    point+=lastlen-1;
    data[point] = x;
    while(point!=0){
      point=(point-1)/2;
      data[point]=func(data[2*point+1],data[2*point+2]);
    }
  }
  T query(ll a,ll b,ll point=0,ll left=0,ll right=-1){
    if(right<0)right=lastlen;
    T ret = iden;
    if(b <= left || right <= a);
    else if(a <= left && right <= b ){
      ret = func(ret, data[point]);
    }
    else{
      T p,q;
      p = query(a,b,point*2+1,left, (left+right)/2);
      q = query(a,b,point*2+2,(left+right)/2, right);
      ret = func(ret,p);
      ret = func(ret,q);
    }      
    return ret;
  }
  T operator[](ll x){
    return data[lastlen-1+x];
  }
  void print(){
    for(ll i=0;i<n;i++){
      if(i!=0)cout<<" ";
      cout<<data[lastlen-1+i];
    }
    cout<<endl;
  }
};
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  //cout<<fixed<<setprecision(10);
  ll res=0,buf=0;
  bool judge = true;
  ll n,m;cin>>n>>m;
  vector<pair<P,ll>>rl(m);
  rep(i,0,m){
    cin>>rl[i].fi.se>>rl[i].fi.fi>>rl[i].se;
    rl[i].fi.se--;
    rl[i].fi.fi--;
  }
  sort(ALL(rl));
  SegmentTree<ll>segtree(vector<ll>(n,LLONG_MAX));
  segtree.update(0,0);
  rep(i,0,m){
    ll l=rl[i].fi.se,r=rl[i].fi.fi,c=rl[i].se;
    ll x=segtree.query(l,r);
    ll p=segtree.query(r,r+1);
    if(x==LLONG_MAX)continue;
    if(x+c<p){
      segtree.update(r,x+c);
    }
  }
  res=segtree.query(n-1,n);
  //cout<<res<<endl;
  if(res==LLONG_MAX)cout<<-1<<endl;
  else cout<<res<<endl;
  return 0;
}

Measured memory_bytes: 5376