#define DEBUG 0
/**
 * File    : D.cpp
 * Author  : Kazune Takahashi
 * Created : 11/9/2019, 10:48:55 PM
 * Powered by Visual Studio Code
 */
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <vector>
#include <string>
#include <complex>
#include <tuple>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <functional>
#include <random>
#include <chrono>
#include <cctype>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
using namespace std;
template <typename T>
void ch_max(T &left, T right)
{
  if (left < right)
  {
    left = right;
  }
}
template <typename T>
void ch_min(T &left, T right)
{
  if (left > right)
  {
    left = right;
  }
}
using ll = long long;
constexpr ll MOD{1000000007LL};
constexpr ll MAX_SIZE{3000010LL};
template <ll MOD = MOD>
class Mint
{
public:
  ll x;
  Mint() : x{0LL} {}
  Mint(ll x) : x{x % MOD} {}
  Mint operator-() const { return x ? MOD - x : 0; }
  Mint &operator+=(const Mint &a)
  {
    if ((x += a.x) >= MOD)
    {
      x -= MOD;
    }
    return *this;
  }
  Mint &operator-=(const Mint &a) { return *this += -a; }
  Mint &operator*=(const Mint &a)
  {
    (x *= a.x) %= MOD;
    return *this;
  }
  Mint &operator/=(const Mint &a)
  {
    Mint b{a};
    return *this *= b.power(MOD - 2);
  }
  Mint operator+(const Mint &a) const { return Mint(*this) += a; }
  Mint operator-(const Mint &a) const { return Mint(*this) -= a; }
  Mint operator*(const Mint &a) const { return Mint(*this) *= a; }
  Mint operator/(const Mint &a) const { return Mint(*this) /= a; }
  bool operator<(const Mint &a) const { return x < a.x; }
  bool operator==(const Mint &a) const { return x == a.x; }
  const Mint power(ll N)
  {
    if (N == 0)
    {
      return 1;
    }
    else if (N % 2 == 1)
    {
      return *this * power(N - 1);
    }
    else
    {
      Mint half = power(N / 2);
      return half * half;
    }
  }
};
template <ll MOD>
Mint<MOD> operator+(ll lhs, const Mint<MOD> &rhs)
{
  return rhs + lhs;
}
template <ll MOD>
Mint<MOD> operator-(ll lhs, const Mint<MOD> &rhs)
{
  return -rhs + lhs;
}
template <ll MOD>
Mint<MOD> operator*(ll lhs, const Mint<MOD> &rhs)
{
  return rhs * lhs;
}
template <ll MOD>
Mint<MOD> operator/(ll lhs, const Mint<MOD> &rhs)
{
  return Mint<MOD>{lhs} / rhs;
}
template <ll MOD>
istream &operator>>(istream &stream, Mint<MOD> &a)
{
  return stream >> a.x;
}
template <ll MOD>
ostream &operator<<(ostream &stream, const Mint<MOD> &a)
{
  return stream << a.x;
}
template <ll MOD = MOD, ll MAX_SIZE = MAX_SIZE>
class Combination
{
public:
  vector<Mint<MOD>> inv, fact, factinv;
  Combination() : inv(MAX_SIZE), fact(MAX_SIZE), factinv(MAX_SIZE)
  {
    inv[1] = 1;
    for (auto i = 2LL; i < MAX_SIZE; i++)
    {
      inv[i] = (-inv[MOD % i]) * (MOD / i);
    }
    fact[0] = factinv[0] = 1;
    for (auto i = 1LL; i < MAX_SIZE; i++)
    {
      fact[i] = Mint<MOD>(i) * fact[i - 1];
      factinv[i] = inv[i] * factinv[i - 1];
    }
  }
  Mint<MOD> operator()(int n, int k)
  {
    if (n >= 0 && k >= 0 && n - k >= 0)
    {
      return fact[n] * factinv[k] * factinv[n - k];
    }
    return 0;
  }
  Mint<MOD> catalan(int x, int y)
  {
    return (*this)(x + y, y) - (*this)(x + y, y - 1);
  }
};
// for C++14
using mint = Mint<MOD>;
using combination = Combination<MOD, MAX_SIZE>;
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
// constexpr double epsilon = 1e-10;
constexpr ll infty = 1000000000000000LL;
// constexpr int dx[4] = {1, 0, -1, 0};
// constexpr int dy[4] = {0, 1, 0, -1};
void Yes()
{
  cout << "Yes" << endl;
  exit(0);
}
void No()
{
  cout << "-1" << endl;
  exit(0);
}

template <typename T>
class SegTree
{ // 0-indexed, [0, N).
private:
  int N;
  vector<T> dat;
  T unit;  // モノイドの単位元
  T(*func) // モノイドの演算
  (T, T);
  T(*_update) // update で値をどうするか書く
  (T, T);

public:
  SegTree() {}

  SegTree(int n, T unit, T (*func)(T, T), T (*_update)(T, T)) : N{1}, unit{unit}, func{func}, _update{_update}
  {
    while (N < n)
    {
      N *= 2;
    }
    dat = vector<T>(2 * N - 1, unit);
  }

  void update(int k, T a)
  {
    k += N - 1;
    dat[k] = _update(dat[k], a);
    while (k > 0)
    {
      k = (k - 1) / 2;
      dat[k] = func(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
  }

private:
  T find(int a, int b, int k, int l, int r)
  {
    if (r <= a || b <= l)
    {
      return unit;
    }
    if (a <= l && r <= b)
    {
      return dat[k];
    }
    T vl = find(a, b, k * 2 + 1, l, (l + r) / 2);
    T vr = find(a, b, k * 2 + 2, (l + r) / 2, r);
    return func(vl, vr);
  }

public:
  T find(int a, int b)
  { // [a, b) の find をする。
    return find(a, b, 0, 0, N);
  }
};

using Info = tuple<int, int, ll>;

int main()
{
  int N, M;
  cin >> N >> M;
  vector<Info> I(M);
  for (auto i = 0; i < M; i++)
  {
    int l, r;
    ll c;
    cin >> l >> r >> c;
    l--;
    r--;
    I[i] = Info(l, r, c);
  }
  sort(I.begin(), I.end());
  auto func = [](auto x, auto y) {
    return min(x, y);
  };
  auto _update = [](auto x, auto y) {
    return min(x, y);
  };
  constexpr ll unit{1LL << 60};
  SegTree<ll> tree{N, unit, func, _update};
  tree.update(0, 0);
  int now{0};
  for (auto i = 0; i < M; i++)
  {
    int l, r;
    ll c;
    tie(l, r, c) = I[i];
    if (now < l)
    {
      No();
    }
    ch_max(now, r);
  }
  if (now < N - 1)
  {
    No();
  }
  for (auto i = 0; i < M; i++)
  {
    int l, r;
    ll c;
    tie(l, r, c) = I[i];
#if DEBUG == 1
    cerr << "l = " << l << ", r = " << r << ", c = " << c << endl;
#endif
    ll mini{tree.find(l, r)};
    tree.update(r, mini + c);
  }
  cout << tree.find(N - 1, N) << endl;
}

Measured memory_bytes: 3840