#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0;i<n;i++)
template< typename Monoid, typename OperatorMonoid = Monoid >
struct LazySegmentTree {
  using F = function< Monoid(Monoid, Monoid) >;
  using G = function< Monoid(Monoid, OperatorMonoid) >;
  using H = function< OperatorMonoid(OperatorMonoid, OperatorMonoid) >;

  long long sz, height;
  vector< Monoid > data;
  vector< OperatorMonoid > lazy;
  const F f;
  const G g;
  const H h;
  const Monoid M1;
  const OperatorMonoid OM0;


  LazySegmentTree(long long n, const F f, const G g, const H h,
                  const Monoid &M1, const OperatorMonoid OM0)
      : f(f), g(g), h(h), M1(M1), OM0(OM0) {
    sz = 1;
    height = 0;
    while(sz < n) sz <<= 1, height++;
    data.assign(2 * sz, M1);
    lazy.assign(2 * sz, OM0);
  }

  void set(long long k, const Monoid &x) {
    data[k + sz] = x;
  }

  void build() {
    for(int k = sz - 1; k > 0; k--) {
      data[k] = f(data[2 * k + 0], data[2 * k + 1]);
    }
  }

  inline void propagate(long long k) {
    if(lazy[k] != OM0) {
      lazy[2 * k + 0] = h(lazy[2 * k + 0], lazy[k]);
      lazy[2 * k + 1] = h(lazy[2 * k + 1], lazy[k]);
      data[k] = reflect(k);
      lazy[k] = OM0;
    }
  }

  inline Monoid reflect(long long k) {
    return lazy[k] == OM0 ? data[k] : g(data[k], lazy[k]);
  }

  inline void recalc(long long k) {
    while(k >>= 1) data[k] = f(reflect(2 * k + 0), reflect(2 * k + 1));
  }

  inline void thrust(long long k) {
    for(int i = height; i > 0; i--) propagate(k >> i);
  }

  void update(long long a, long long b, const OperatorMonoid &x) {
    thrust(a += sz);
    thrust(b += sz - 1);
    for(int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
      if(l & 1) lazy[l] = h(lazy[l], x), ++l;
      if(r & 1) --r, lazy[r] = h(lazy[r], x);
    }
    recalc(a);
    recalc(b);
  }

  Monoid query(long long a, long long b) {
    thrust(a += sz);
    thrust(b += sz - 1);
    Monoid L = M1, R = M1;
    for(int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
      if(l & 1) L = f(L, reflect(l++));
      if(r & 1) R = f(reflect(--r), R);
    }
    return f(L, R);
  }

  Monoid operator[](const long long &k) {
    return query(k, k + 1);
  }

  template< typename C >
  long long find_subtree(long long a, const C &check, Monoid &M, bool type) {
    while(a < sz) {
      propagate(a);
      Monoid nxt = type ? f(reflect(2 * a + type), M) : f(M, reflect(2 * a + type));
      if(check(nxt)) a = 2 * a + type;
      else M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }

  template< typename C >
  long long find_first(long long a, const C &check) {
    Monoid L = M1;
    if(a <= 0) {
      if(check(f(L, reflect(1)))) return find_subtree(1, check, L, false);
      return -1;
    }
    thrust(a + sz);
    long long b = sz;
    for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if(a & 1) {
        Monoid nxt = f(L, reflect(a));
        if(check(nxt)) return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }


  template< typename C >
  long long find_last(long long b, const C &check) {
    Monoid R = M1;
    if(b >= sz) {
      if(check(f(reflect(1), R))) return find_subtree(1, check, R, true);
      return -1;
    }
    thrust(b + sz - 1);
    long a = sz;
    for(b += sz; a < b; a >>= 1, b >>= 1) {
      if(b & 1) {
        Monoid nxt = f(reflect(--b), R);
        if(check(nxt)) return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};
int main(){
    int n,m;
    cin >> n >> m;
    pair<pair<int,int>,long long> pr[m];
    rep(i,m){
        cin >> pr[i].first.second >> pr[i].first.first >> pr[i].second;
        pr[i].first.second--;
        pr[i].first.first--;
    }
    sort(pr,pr+m);
    LazySegmentTree< long long > seg(n, [](long long a, long long b) { return min(a, b); }, [](long long a, long long b) { return min(a, b); },[](long long a, long long b) { return min(a, b); },1e15,1e15);
    seg.update(0,1,0LL);
    //cout << endl;
    rep(i,m){
        long long tmp = seg.query(pr[i].first.second,1+pr[i].first.first);
        seg.update(pr[i].first.second,1+pr[i].first.first,tmp+pr[i].second);
        //cout << pr[i].first.second << " " << pr[i].first.first << " " << pr[i].second << " " << tmp << "   ";
        //rep(j,n) cout << seg.node[15+j] << " ";//seg.find(j,j+1) << " ";//seg.node[15+j] << " ";
        //cout << endl; 
    }
    long long tm = -1;
    if(seg.query(n-1,n)==1e15) cout << -1 << endl;
    else cout << seg.query(n-1,n) << endl;
}

Measured memory_bytes: 5888