#include <bits/stdc++.h>
using namespace std;
using vint = vector<long long>;
using vvint = vector<vint>;
using pint = pair<long long, long long>;
using vpint = vector<pint>;
template <typename T>
using priority_queue_rev = priority_queue<T, vector<T>, greater<T>>;
constexpr long double pi = 3.1415926535897932384626433832795028;
constexpr long long dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
constexpr long long dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
long long gcd(long long a, long long b) {
  while (b) {
    swap(a %= b, b);
  }
  return a;
}
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
template <typename T1, typename T2>
bool chmax(T1 &a, const T2 &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
bool chmin(T1 &a, const T2 &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &rhs) {
  os << "(" << rhs.first << ", " << rhs.second << ")";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &rhs) {
  os << "{";
  for (auto itr = rhs.begin(); itr != rhs.end(); itr++) {
    os << *itr << (asdnext(itr) != rhs.end() ? ", " : "");
  }
  os << "}";
  return os;
}
struct setup {
  static constexpr long long PREC = 20;
  setup() {
    cout << fixed << setprecision(PREC);
    cerr << fixed << setprecision(PREC);
  };
} setup;
template <typename T>
struct segment_tree {
  long long n;
  T ZERO;
  std::vector<T> data;
  long long get_size(long long n_) const {
    long long ret = 1;
    while (ret < n_) {
      ret <<= 1;
    }
    return ret;
  }
  segment_tree(long long n_, T ZERO = T())
      : n(get_size(n_)), ZERO(ZERO), data(n << 1, ZERO) {}
  template <typename I>
  segment_tree(I begin, I end, T ZERO = T())
      : n(get_size(end - begin)), ZERO(ZERO), data(n << 1) {
    std::copy(begin, end, data.begin() + n);
    for (long long i = n - 1; i > 0; i--) {
      data[i] = data[i << 1] + data[(i << 1) + 1];
    }
  }
  void set(long long i, T x) {
    i += n;
    data[i] = x;
    while (i > 1) {
      i >>= 1;
      data[i] = data[i << 1] + data[(i << 1) + 1];
    }
  }
  T operator[](long long i) const { return data[i + n]; }
  T get_sum(long long l, long long r) const {
    if (l < 0) {
      l = 0;
    }
    if (r > n) {
      r = n;
    }
    T ret = ZERO;
    for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
      if (l & 1) {
        ret += data[l], l++;
      }
      if (r & 1) {
        ret += data[r - 1];
      }
    }
    return ret;
  }
};
struct min_monoid {
  long long val;
  min_monoid() : val(LLONG_MAX) {}
  min_monoid(long long val) : val(val) {}
  bool operator==(const min_monoid &rhs) const { return val == rhs.val; }
  bool operator!=(const min_monoid &rhs) const {
    return std::rel_ops::operator!=(*this, rhs);
  }
  min_monoid &operator+=(const min_monoid &rhs) {
    val = std::min(val, rhs.val);
    return *this;
  }
  min_monoid operator+(const min_monoid rhs) const {
    return min_monoid(*this) += rhs;
  }
  friend std::istream &operator>>(std::istream &is, min_monoid &rhs) {
    long long v;
    is >> v;
    rhs = min_monoid(v);
    return is;
  }
  friend std::ostream &operator<<(std::ostream &os, const min_monoid &rhs) {
    return os << rhs.val;
  }
};
long long N, M;
long long L[110000], R[110000], C[110000];
pair<pint, long long> RLC[110000];
signed main() {
  cin >> N >> M;
  for (long long i = 0; i < (M); i++) {
    cin >> L[i] >> R[i] >> C[i];
    RLC[i] = {{R[i], L[i]}, C[i]};
  }
  sort(RLC, RLC + M);
  segment_tree<min_monoid> rmq(N + 1);
  rmq.set(1, 0);
  for (long long i = 0; i < (M); i++) {
    long long l = RLC[i].first.second, r = RLC[i].first.first,
              c = RLC[i].second;
    rmq.set(r, rmq[r] + min_monoid(rmq.get_sum(l, r).val + c));
  }
  cout << ((rmq[N] == LLONG_MAX || rmq[N].val < 0) ? -1 : rmq[N]) << endl;
}
