/* monkukui 競技プログラミング用のテンプレート  (ここから) */
#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <deque>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <complex>
#include <cmath>
#include <limits>
#include <climits>
#include <ctime>
#include <cassert>
#include <numeric>
#include <functional>
#include <bitset>

using namespace std;
using lint = long long int;
using ll = long long int;
using lnt = long long int;
using Graph = vector<vector<int>>;
using Wgraph = vector<vector<pair<int, long long int>>>;
constexpr long long int INF = 1001001001001001LL;
constexpr long long int MOD = 1000000007LL;
constexpr double PI = 3.1415926535897932;
constexpr long long int di[] = {-1, 0, 1, 0, -1, 1, 1, -1};
constexpr long long int dj[] = {0, 1, 0, -1, 1, 1, -1, -1};

template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}
inline void yes(){ cout << "yes" << endl; }
inline void Yes(){ cout << "Yes" << endl; }
inline void YES(){ cout << "YES" << endl; }
inline void no(){ cout << "no" << endl; }
inline void No(){ cout << "No" << endl; }
inline void NO(){ cout << "NO" << endl; }
inline void possible(){ cout << "possible" << endl; }
inline void Possible(){ cout << "Possible" << endl; }
inline void POSSIBLE(){ cout << "POSSIBLE" << endl; }
inline void impossible(){ cout << "impossible" << endl; }
inline void Impossible(){ cout << "Impossible" << endl; }
inline void IMPOSSIBLE(){ cout << "IMPOSSIBLE" << endl; }

#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 SREP(i,s,t) for(int i = s; i < t; i++)
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define PB push_back

/* monkukui 競技プログラミング用のテンプレート (ここまで)*/


// quoted from beet-aizu
template <typename T,typename E, typename F, typename G, typename H>
struct LazySegmentTree{
    //using F = function<T(T,T)>;
    //using G = function<T(T,E)>;
    //using H = function<E(E,E)>;
    int n,height;
    F f;
    G g;
    H h;
    T ti;
    E ei;
    vector<T> dat;
    vector<E> laz;
    LazySegmentTree(F f,G g,H h,T ti,E ei):
        f(f),g(g),h(h),ti(ti),ei(ei){}

    void init(int n_){
        n=1;height=0;
        while(n<n_) n<<=1,height++;
        dat.assign(2*n,ti);
        laz.assign(2*n,ei);
    }
    void build(const vector<T> &v){
        int n_=v.size();
        init(n_);
        for(int i=0;i<n_;i++) dat[n+i]=v[i];
        for(int i=n-1;i;i--)
            dat[i]=f(dat[(i<<1)|0],dat[(i<<1)|1]);
    }
    inline T reflect(int k){
        return laz[k]==ei?dat[k]:g(dat[k],laz[k]);
    }
    inline void eval(int k){
        if(laz[k]==ei) return;
        laz[(k<<1)|0]=h(laz[(k<<1)|0],laz[k]);
        laz[(k<<1)|1]=h(laz[(k<<1)|1],laz[k]);
        dat[k]=reflect(k);
        laz[k]=ei;
    }
    inline void thrust(int k){
        for(int i=height;i;i--) eval(k>>i);
    }
    inline void recalc(int k){    
        while(k>>=1)
            dat[k]=f(reflect((k<<1)|0),reflect((k<<1)|1));
    }
    void update(int a,int b,E x){
        thrust(a+=n);
        thrust(b+=n-1);
        for(int l=a,r=b+1;l<r;l>>=1,r>>=1){
            if(l&1) laz[l]=h(laz[l],x),l++;
            if(r&1) --r,laz[r]=h(laz[r],x);
        }
        recalc(a);
        recalc(b);
    }
    void set_val(int a,T x){
        thrust(a+=n);
        dat[a]=x;laz[a]=ei;
        recalc(a);
    }
    T query(int a,int b){
        thrust(a+=n);
        thrust(b+=n-1);
        T vl=ti,vr=ti;
        for(int l=a,r=b+1;l<r;l>>=1,r>>=1) {
            if(l&1) vl=f(vl,reflect(l++));
            if(r&1) vr=f(reflect(--r),vr);
        }
        return f(vl,vr);
    }
};

int main(){
  using T = lint;
  using E = lint;
  auto f = [](T a, T b){ // return type T value
    return min(a, b);
  };
  auto g = [](T a, E b){ // return type T value
    return min(a, b);
  };
  auto h = [](E a, E b){ // return type E value
    return min(a, b);
  };
  T ti = INF;  // identify element
  E ei = INF;  // identify element
  LazySegmentTree<T, E, decltype(f), decltype(g), decltype(h)> sg(f, g, h, ti, ei);  // don't change
  lint n, m; cin >> n >> m;
  sg.build(vector<lint> (n, INF));   // 初期配列を代入
  sg.update(0, 1, 0);

  vector<pair<pair<lint, lint>, lint>> edges(m);
  for(int i = 0; i < m; i++) {
    int l, r, c; cin >> l >> r >> c;
    l--;
    r--;
    edges[i] = {{l, r}, c};
  }

  sort(ALL(edges));

  for(int i = 0; i < m; i++) {
    lint l = edges[i].first.first;
    lint r = edges[i].first.second;
    lint cost = edges[i].second;
    lint curCost = sg.query(l, l + 1);
    sg.update(l, r + 1, curCost + cost);
  }

  if(sg.query(n - 1, n) == INF) cout << -1 << endl;
  else cout << sg.query(n - 1, n) << endl;
}


Measured memory_bytes: 6676