#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class Monoid>
struct SegmentTree {
 private:
  using T = typename Monoid::T;
  using size_type = std::size_t;
  size_type n;
  std::vector<T> data;
  void prop(int i) { data[i] = Monoid::op(data[2 * i], data[2 * i + 1]); }

 public:
  SegmentTree() : n(0) {}
  SegmentTree(int n, T initial = Monoid::identity()) : n(n) {
    data.resize(n * 2, initial);
    for (int i = n - 1; i > 0; i--)
      data[i] = Monoid::op(data[i * 2], data[i * 2 + 1]);
  }
  template <class InputIter,
            class = typename std::iterator_traits<InputIter>::value_type>
  SegmentTree(InputIter first, InputIter last)
      : SegmentTree(distance(first, last)) {
    copy(first, last, data.begin() + n);
    for (int i = n - 1; i > 0; i--) prop(i);
  }
  SegmentTree(std::vector<T> v) : SegmentTree(v.begin(), v.end()) {}
  SegmentTree(std::initializer_list<T> v) : SegmentTree(v.begin(), v.end()) {}
  void set(size_t i, const T &v) {
    assert(i < n);
    data[i += n] = v;
    while (i >>= 1) prop(i);
  }
  T get(size_t i) {
    assert(i < n);
    return data[i + n];
  }
  T fold(int l, int r) {
    if (l < 0) l = 0;
    if (l >= r) return Monoid::identity();
    if (r > static_cast<int>(n)) r = n;
    T tmpL = Monoid::identity(), tmpR = Monoid::identity();
    for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
      if (l & 1) tmpL = Monoid::op(tmpL, data[l++]);
      if (r & 1) tmpR = Monoid::op(data[--r], tmpR);
    }
    return Monoid::op(tmpL, tmpR);
  }
  size_type size() { return n; }
  inline void dum(int r = -1) {}
};
constexpr long long inf_monoid = 1e18 + 100;
struct Nothing {
  using T = char;
  using Monoid = Nothing;
  using M = T;
  static constexpr T op(const T &, const T &) { return T(); }
  static constexpr T identity() { return T(); }
  template <class X>
  static constexpr X actInto(const M &, long long, const X &x) {
    return x;
  }
};
template <class U = long long>
struct RangeMin {
  using T = U;
  static T op(const T &a, const T &b) { return std::min<T>(a, b); }
  static constexpr T identity() { return T(inf_monoid); }
};
template <class U = long long>
struct RangeMax {
  using T = U;
  static T op(const T &a, const T &b) { return std::max<T>(a, b); }
  static constexpr T identity() { return T(-inf_monoid); }
};
template <class U = long long>
struct RangeSum {
  using T = U;
  static T op(const T &a, const T &b) { return a + b; }
  static constexpr T identity() { return T(0); }
};
template <class U>
struct RangeProd {
  using T = U;
  static T op(const T &a, const T &b) { return a * b; }
  static constexpr T identity() { return T(1); }
};
template <class U = long long>
struct RangeOr {
  using T = U;
  static T op(const T &a, const T &b) { return a | b; }
  static constexpr T identity() { return T(0); }
};
template <class U = long long>
struct RangeAnd {
  using T = U;
  static T op(const T &a, const T &b) { return a & b; }
  static constexpr T identity() { return T(-1); }
};
template <size_t N>
struct RangeAnd<std::bitset<N>> {
  using T = std::bitset<N>;
  static T op(const T &a, const T &b) { return a & b; }
  static constexpr T identity() { return std::bitset<N>().set(); }
};
using Seg = SegmentTree<RangeMin<>>;
int l[112345], r[112345], c[112345];
int main() {
  std::ios::sync_with_stdio(false), std::cin.tie(0);
  int n, m;
  cin >> n >> m;
  map<int, vector<int>> mp;
  for (int i = 0; i < m; i++) {
    cin >> l[i] >> r[i] >> c[i];
    mp[r[i]].push_back(i);
  }
  constexpr int inf = 1e9 + 1;
  Seg dp(n + 1, inf);
  dp.set(1, 0);
  for (int i = 2; i <= n; i++) {
    for (auto j : mp[i]) {
      dp.set(i, min<ll>(dp.get(i), dp.fold(l[j], i) + c[j]));
    }
  }
  ll ans = dp.get(n);
  if (ans >= inf) ans = -1;
  cout << ans << endl;
  return 0;
}
