#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
template<typename Monoid> class SegmentTree {
private:
  using Func = function<Monoid(Monoid, Monoid)>;
  Func func;
  Monoid unity;
  int siz;
  vector<Monoid> data;
  int n;
  int height;
  inline void build() {
    siz = 1;
    while (siz < n) siz <<= 1, ++height;
    data.assign(siz << 1, unity);
    for (int i = 0; i < n; i++) set(i, unity);
    for (int idx = siz - 1; idx > 0; idx--) data[idx] = func(data[(idx << 1) | 0], data[(idx << 1) | 1]);
  }
  template<typename T> inline void build(const T& arr) {
    if (!~n) n = (int) arr.size();
    siz = 1;
    while (siz < n) siz <<= 1, ++height;
    data.assign(siz << 1, unity);
    for (int i = 0; i < n; i++) set(i, arr[i]);
    for (int idx = siz - 1; idx > 0; idx--) data[idx] = func(data[(idx << 1) | 0], data[(idx << 1) | 1]);
  }
  inline void set(int idx, const Monoid& val) { assert(idx + siz < (int) data.size()); data[idx + siz] = val;}

public:
  SegmentTree() {}
  SegmentTree (const Func f, const Monoid& u, const int& n_) : func(f), unity(u), n(n_) { build(); }
  template<typename T> SegmentTree (const T& arr, const Func f, const Monoid& u, int n_ = -1) : func(f), unity(u), n(n_) { build(arr); }
  

  void initialize (const Func f, const Monoid &u, const int& n_) { func = f; unity = u; n = n_; build(); }
  template<typename T> void initialize (const T& arr, const Func f, const Monoid& u, int n_ = -1) { func = f; unity = u; n = n_; buld(arr); }

  void modify (int idx, const Monoid& val) {
    idx += siz;
    data[idx] = val;
    while (idx >>= 1) data[idx] = func(data[(idx << 1) | 0], data[(idx << 1) | 1]);
  }
  //[left, right) 
  Monoid get (int left, int right) {
    if (left > right) swap(left, right);
    Monoid val_left = unity, val_right = unity;
    for (int l = left + siz, r = right + siz; l < r; l >>= 1, r >>= 1) {
      if (l & 1) val_left = func(val_left, data[l++]);
      if (r & 1) val_right = func(data[--r], val_right);
    }
    return func(val_left, val_right);
  }

  template<typename U>
  int find (int start, int left, int right, int cur, Monoid& val, const U& f) {
    if (left + 1 == right) {
      val = func(val, data[cur]);
      return f(val) ? cur - siz : -1;
    }
    int mid = (left + right) >> 1;
    if (mid <= start) return find(start, mid, right, (cur << 1) | 1, val, f);
    if (start <= left && !f(func(val, data[cur]))) {
      val = func(val, data[cur]);
      return -1;
    }
    int val_left = find(start, left, mid, (cur << 1) | 0, val, f);
    if (~val_left) return val_left;
    return find(start, mid, right, (cur << 1) | 1, val, f);
  }
  template<typename U>
  int find (int start, const U& f) {
    Monoid val = unity;
    return find(start, 0, siz, 1, val, f);
  }

  inline Monoid operator[] (int idx) { return data[idx + siz]; }
  void print() {
    for (int i = 0; i < siz; i++) {
      cout << (*this)[i];
      if (i != siz - 1) cout << ", ";
    }
    cout << '\n';
  }
};

// long long max
function<long long(long long, long long)> LLMAX = [] (long long a, long long b) -> long long { return max(a, b); };
const long long LLMAX_UNITY = numeric_limits<long long>::min();
// int max
function<int(int, int)> INTMAX = [] (int a, int b) -> int { return max(a, b); };
const int INTMAX_UNITY = numeric_limits<int>::min();
// long long min
function<long long(long long, long long)> LLMIN = [] (long long a, long long b) -> long long { return min(a, b); };
const long long LLMIN_UNITY = numeric_limits<long long>::max();
// int min
function<int(int, int)> INTMIN = [] (int a, int b) -> int { return min(a, b); };
const int INTMIN_UNITY = numeric_limits<int>::max();
// long long sum
function<long long(long long, long long)> LLSUM = [] (long long a, long long b) -> long long { return a + b; };
const long long LLSUM_UNITY = 0LL;
// int sum
function<int(int, int)> INTSUM = [] (int a, int b) -> int { return a + b; };
const int INTSUM_UNITY = 0;

signed main() { 
  ios::sync_with_stdio(false); cin.tie(0);
  //d
  int n, m;
  cin >> n >> m;
  vector<vector<long long>> a;
  for (int i = 0; i < m; i++) {
    long long l, r, c;
    cin >> l >> r >> c;
    a.push_back({--l, --r, c});
  }
  sort(a.begin(), a.end());
  SegmentTree<long long> sgt(LLMIN, LLMIN_UNITY / 3, n + 1);
  sgt.modify(0, 0);
  for (int i = 0; i < m; i++) {
    long long min_from = sgt.get(a[i][0], a[i][1] + 1);
    long long cur = sgt.get(a[i][1], a[i][1] + 1);
    if (cur > min_from + a[i][2]) sgt.modify(a[i][1], min_from + a[i][2]);
  }
  //cerr << sgt.get(n - 1, n) << endl;
  cout << (sgt.get(n - 1, n) < LLMIN_UNITY/ 3 ? sgt.get(n - 1, n) : -1) << '\n';
  return 0;
}

Measured memory_bytes: 7984