#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i,n) for(int (i)=((n)-1);(i)>=0;(i)--)
#define itn int
#define miele(v) min_element(v.begin(), v.end())
#define maele(v) max_element(v.begin(), v.end())
#define SUM(v) accumulate(v.begin(), v.end(), 0LL)
#define lb(a, key) lower_bound(a.begin(),a.end(),key)
#define ub(a, key) upper_bound(a.begin(),a.end(),key)
#define COUNT(a, key) count(a.begin(), a.end(), key) 
#define BITCOUNT(x) __builtin_popcount(x)
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
using P = pair <int,int>;
using WeightedGraph = vector<vector <P>>;
using UnWeightedGraph = vector<vector<int>>;
using Real = long double;
using Point = complex<Real>; //Point and Vector2d is the same!
using Vector2d = complex<Real>;
const long long INF = 1LL << 60;
const int MOD = 1000000007;
const double EPS = 1e-15;
const double PI=3.14159265358979323846;
template <typename T> 
int getIndexOfLowerBound(vector <T> &v, T x){
    return lower_bound(v.begin(),v.end(),x)-v.begin();
}
template <typename T> 
int getIndexOfUpperBound(vector <T> &v, T x){
    return upper_bound(v.begin(),v.end(),x)-v.begin();
}
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
#define DUMPOUT cerr
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
istream &operator>>(istream &is, Point &p) {
    Real a, b; is >> a >> b; p = Point(a, b); return is;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T,U> &p_var) {
    is >> p_var.first >> p_var.second;
    return is;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}
template <typename T>
istream &operator>>(istream &is, vector<vector<T>> &df) {
    for(int i=0;i<df.size();i++)for(int j=0;j<df[i].size();j++)
    is >> df[i][j];
    return is;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    DUMPOUT<<'{';
    os << pair_var.first;
    DUMPOUT<<',';
    os << " "<< pair_var.second;
    DUMPOUT<<'}';
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
    DUMPOUT<<'[';
    for (int i = 0; i < vec.size(); i++) 
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
    DUMPOUT<<']';
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<vector<T>> &df) {
  for (auto& vec : df) os<<vec;
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    DUMPOUT << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) DUMPOUT << ", ";
        itr--;
    }
    DUMPOUT << "}";
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    DUMPOUT << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) DUMPOUT << ", ";
        itr--;
    }
    DUMPOUT << "}";
    return os;
}
void print() {cout << endl;}
template <class Head, class... Tail>
void print(Head&& head, Tail&&... tail) {
  cout << head;
  if (sizeof...(tail) != 0) cout << " ";
  print(forward<Tail>(tail)...);
}
void dump_func() {DUMPOUT << '#'<<endl;}
template <typename Head, typename... Tail>
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) DUMPOUT << ", ";
    dump_func(std::move(tail)...);
}
#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct LazySegTree{ 
    int n;
    vector <int> node, lazy;
   
    LazySegTree(vector <int> v){
        int sz = v.size();
        n = 1;
        while(n<sz) n*=2;
        
        node.resize(2*n-1, INF);
        lazy.resize(2*n-1, INF);
        
        for(int i=0; i<sz; i++) node[i+n-1] = v[i]; 
        for(int i=n-2;i>=0;i--) node[i] = min(node[i * 2 + 1], node[i * 2 + 2]);
    }
 
    void eval(int k,int l, int r){
        if(lazy[k] != INF){
            chmin(node[k],lazy[k]);
            if(r - l >1){
                chmin(lazy[2*k+1] , lazy[k]);
                chmin(lazy[2*k+2] , lazy[k]);
            }
            lazy[k] = INF;
        }
    }
 
    void minimize(int a, int b, int x, int k=0, int l=0, int r=-1) {
        //区間に足すのではなくminで更新する
        if(r < 0) r = n;
        eval(k, l, r);
        if(b <= l || r <= a) return;
        if(a <= l && r <= b) {
            lazy[k] = x;
            eval(k, l, r);
        }
        else {
            minimize(a, b, x, 2*k+1, l, (l+r)/2);
            minimize(a, b, x, 2*k+2, (l+r)/2, r);
            node[k] = min(node[2*k+1] , node[2*k+2]);    
        }
    }
    int getmin(int a, int b, int k=0, int l=0, int r=-1) {
        
        if(r < 0) r = n;
        if(b <= l || r <= a) return INF;
        eval(k, l, r);
        
        if(a <= l && r <= b) {
            //cout<<node[k]<<"+"<<endl;
            return node[k];
        }
        int vl = getmin(a, b, 2*k+1, l, (l+r)/2);
        int vr = getmin(a, b, 2*k+2, (l+r)/2, r);
        
        //cout<<vl<<' '<< l<<' '<<r<<endl<<vr<<'+'<<(l+r)/2<<'+'<<r<<endl;
        return min(vl, vr);
    }
};
signed main(void) { cin.tie(0); ios::sync_with_stdio(false);
    int n, m; cin>>n>>m;
    WeightedGraph g(n);
    vector <tuple <int,int,int>> edges(m);
    rep(i, m){
        int l, r, c; cin>>l>>r>>c;
        l--, r--;
        edges[i] = make_tuple(l, r, c);
    }
    sort(edges.begin(),edges.end());
    vector <int> tmp(n, INF);
    LazySegTree Lseg(tmp);
    Lseg.minimize(0, 1, 0);
    rep(i, m){
        int l = get <0>(edges[i]), r = get <1>(edges[i]), c = get <2>(edges[i]);
        int mi = Lseg.getmin(l, r+1);
        dump(mi, c);
        Lseg.minimize(l, r+1, mi + c);
    }
    int ans = Lseg.getmin(n-1, n);
    if(ans >= INF) print(-1); else print(ans);
}


Measured memory_bytes: 10496