#include <bits/stdc++.h>
#include <unistd.h>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
 
 
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
//const ll MOD = 1000000007;
const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx_ori[4] = { -1,0, 1,0 }, dy_ori[4] = { 0,1,0,-1 };
int dx[4] = {0, 0, 0, 0}, dy[4] = {0, 0, 0, 0};
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
// #define mp make_pair
//#define endl '\n'


template <typename T1, typename T2>
class LazySegmentTree {
public:
  using F = function<T1(T1&, T1&)>; // query_func
  using G = function<T2(T2&, T2&)>; // update_func
  using H = function<T1(T1&, T2&, int, int)>; // lazy to node (node, lazy, left, right)
  int n;
  vector<T1> node;
  vector<T2> lazy;
  T1 e1;
  T2 e2;
  F query_func;
  G update_func;
  H eval_func;

  LazySegmentTree(vector<T1> a, F query_func, G update_func, H eval_func, T1 e1, T2 e2)
    :query_func(query_func), update_func(update_func), eval_func(eval_func), e1(e1), e2(e2)
  {
    int _n = a.size();
    n = 1; while (n < _n) n *= 2;
    node.resize(2 * n - 1, e1);
    lazy.resize(2 * n - 1, e2);
    for (int i = 0; i < _n; i++) node[i + n - 1] = a[i];
    for (int i = n - 2; i >= 0; i--) {
      node[i] = query_func(node[i * 2 + 1], node[i * 2 + 2]);
    }
  }

  // k番目のノードについて遅延評価を行う
  inline void eval(int k, int l, int r) {
    if (lazy[k] != e2) { // Change
      node[k] = eval_func(node[k], lazy[k], l, r);
      if (r - l > 1) {
        lazy[2 * k + 1] = update_func(lazy[2*k + 1], lazy[k]);
        lazy[2 * k + 2] = update_func(lazy[2*k + 2], lazy[k]);
      }
      lazy[k] = e2; // Change
    }
  }

  // [a, b)をxにする
  void update(int a, int b, T2 x, int k, int l, int r) {
    // k 番目のノードに対して遅延評価を行う
    eval(k, l, r);
    if (b <= l || r <= a) return;
    if (a <= l && r <= b) {
      lazy[k] = update_func(lazy[k], x);
      eval(k, l, r);
    }
    else {
      update(a, b, x, 2 * k + 1, l, (l + r) / 2);
      update(a, b, x, 2 * k + 2, (l + r) / 2, r);
      node[k] = query_func(node[2 * k + 1], node[2 * k + 2]);
    }
  }

  T1 query(int a, int b, int k, int l, int r) {
    eval(k, l, r);
    if (b <= l || r <= a) return e1;
    if (a <= l && r <= b) return node[k];
    T1 resl = query(a, b, 2 * k + 1, l, (l + r) / 2);
    T1 resr = query(a, b, 2 * k + 2, (l + r) / 2, r);
    return query_func(resl, resr);
  }
};


ll f1(ll a, ll b) {
  return min(a, b);
}

ll f2(ll a, ll b) {
  return min(a, b);
}

ll f3(ll a, ll b, int l, int r) {
  return min(a, b);
}

vi zaatsu(vi a) {
  int n = a.size();
  set<int> st;
  rep (i, n) st.insert(a[i]);
  map<int, int> mp;
  int cnt = 0;
  for (auto itr = st.begin(); itr != st.end(); itr++) {
    mp[*itr] = cnt++;
  }
  rep (i, n) {
    a[i] = mp[a[i]];
  }
  return a;
}

class UnionFind {
public:
  int n;
  vi par; //親
  vi ran; //木の深さ
  vi num; //要素数
  int g; // group数
 
  UnionFind(int _n) {
    n = _n;
    g = n;
    par.resize(n); ran.resize(n); num.resize(n);
    for (int i = 0; i < n; i++) {
      par[i] = i; ran[i] = 0; num[i] = 1;
    }
  }
 
  //木の根を求める
  int find(int x) {
    if (par[x] == x) {
      return x;
    }
    else {
      return par[x] = find(par[x]);
    }
  }
 
  //xとyの属する集合を併合
  void unite(int x, int y) {
    x = find(x); y = find(y);
    int numsum = num[x] + num[y];
    if (x == y) {
      return;
    }
    if (ran[x]<ran[y]) {
      par[x] = y;
    }
    else {
      par[y] = x;
      if (ran[x] == ran[y]) {
        ran[x]++;
      }
    }
    num[x] = num[y] = numsum;
    g--;
  }
 
  //xとyが同じ集合に属するか否か
  bool same(int x, int y) {
    return find(x) == find(y);
  }
};


signed main() {
  fio();
  int n, m;
  cin >> n >> m;
  vector<pair<pii, ll>> clr(m);
  rep (i, m) {
    int l, r, c;
    cin >> l >> r >> c;
    l--; r--;
    clr[i] = make_pair(pii(l, r), c);
  }
  sort(all(clr));

  LazySegmentTree<ll, ll> seg(vl(n, INF), f1, f2, f3, INF, INF);
  seg.update(0, 1, 0, 0, 0, seg.n);
  rep (i, m) {
    pii lr = clr[i].first;
    ll x = clr[i].second;
    int l = lr.first, r = lr.second;
    int start = seg.query(l, l + 1, 0, 0, seg.n);
    int goal = start + x;
    seg.update(l, r + 1, goal, 0, 0, seg.n);
    /*
    rep (i, n) {
      cout << seg.query(i, i + 1, 0, 0, seg.n) << " ";
    }
    cout << endl;
    //*/
  }
  ll ans = seg.query(n - 1, n, 0, 0, seg.n);
  if (ans >= INF / 2) {
    cout << -1 << endl;
  }
  else {
    cout << ans << endl;
  }
}


Measured memory_bytes: 8596