#include <iostream>
#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep_r(i, n) for (int i = n - 1; i >= 0; i--)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((ll)(x).size())
#define bit(n) (1LL << (n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

#define INF bit(60)
#define pb push_back
#define mod 1000000007

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 (b < a)
  {
    a = b;
    return 1;
  }
  return 0;
}

using namespace std;
using uif = uint_fast64_t;
using ll = long long int;
using tTree = __gnu_pbds::tree<ll, __gnu_pbds::null_type, less<ll>, __gnu_pbds::rb_tree_tag, __gnu_pbds::tree_order_statistics_node_update>;

using pa = pair<ll, ll>;

int qp(int a, ll b)
{
  int ans = 1;
  do
  {
    if (b & 1)
      ans = 1ll * ans * a % mod;
    a = 1ll * a * a % mod;
  } while (b >>= 1);
  return ans;
}
int qp(int a, ll b, int mo)
{
  int ans = 1;
  do
  {
    if (b & 1)
      ans = 1ll * ans * a % mo;
    a = 1ll * a * a % mo;
  } while (b >>= 1);
  return ans;
}

int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

#define FACSIZE 2002

ll invfac[FACSIZE];
ll fac[FACSIZE];

ll mod_pow(ll, ll);
ll mod_add(ll, ll);
ll mod_mul(ll, ll);
ll mod_div(ll, ll);
void make_fact(ll);
void make_invfact(ll);

int main(void)
{
  ll n, m;
  cin >> n >> m;
  vector<ll> L(n);
  vector<ll> R(n);
  vector<ll> C(n);
  rep(i, m) cin >> L[i] >> R[i] >> C[i];

  vector<vector<pa>> vec(n + 1, vector<pa>());
  vector<deque<pa>> dvec(n + 1, deque<pa>());

  //vector<vector<pa>> vecSorted(n + 1, vector<pa>());
  vector<map<ll, ll>> vecSorted(n + 1, map<ll, ll>());

  rep(i, m)
  {
    vec[L[i]].push_back(make_pair(R[i], C[i]));
  }
  rep(i, n + 1)
  {
    sort(begin(vec[i]), end(vec[i]));
  }
  rep(i, n + 1)
  {

    for (auto p : vec[i])
    {
      if (dvec[i].empty())
        dvec[i].push_back(p);
      else
      {
        while (!dvec[i].empty() && dvec[i].back().second > p.second)
        {
          dvec[i].pop_back();
        }
        if (dvec[i].empty() || dvec[i].back().first < p.first)
          dvec[i].push_back(p);
      }
    }
  }

  rep(i, n + 1)
  {
    while (!dvec[i].empty())
    {
      auto q = dvec[i].front();
      //vecSorted[i].push_back(q);
      vecSorted[i][q.first] = q.second;
      dvec[i].pop_front();
    }
  }
  /*
  rep(i, n + 1)
  {
    auto v = vecSorted[i];
    cout << "i:" << i << " ";
    for (auto x : v)
      cout << x.first << "," << x.second << " ";
    cout << endl;
  }
  //*/

  ///*
  //vector<ll> dis(n + 1, LLONG_MAX);
  map<ll, ll> dis;
  dis[1] = 0;
  if (vecSorted[1].empty())
  {
    cout << -1 << endl;
    return 0;
  }
  for (auto p : vecSorted[1])
    dis[p.first] = p.second;
  for (ll i = 2; i <= n; i++)
  {
    for (auto p : vecSorted[i])
    {
      auto itr1 = dis.lower_bound(i);
      if (itr1 == end(dis))
        continue;
      auto itr2 = dis.lower_bound(p.first);
      if (itr2 == end(dis))
      {
        dis[p.first] = p.second + itr1->second;
        auto itrTarget = dis.find(p.first);
      }
      else if (itr2->second > p.second + itr1->second)
      {
        dis[p.first] = p.second + itr1->second;
      }
    }
  }
  /*
  for (auto p : dis)
  {
    cout << p.first << "," << p.second << endl;
  }
  */
  auto itr = dis.lower_bound(n);
  if (itr == end(dis))
  {
    cout << -1 << endl;
    return 0;
  }
  else
    cout << itr->second << endl;

  return 0;
}

ll mod_pow(ll x, ll r)
{
  if (r == 0)
    return 1;
  else if (r == 1)
    return x % mod;
  else if (r % 2 == 0)
  {
    ll t = mod_pow(x, r / 2) % mod;
    return t * t % mod;
  }
  else
  {
    ll t = mod_pow(x, r / 2) % mod;
    ll k = t * t % mod;
    return k * x % mod;
  }
}
ll mod_add(ll a, ll b)
{
  return (a + b) % mod;
}

ll mod_mul(ll a, ll b)
{
  return (a * b) % mod;
}

ll mod_div(ll a, ll b)
{
  return mod_mul(a, mod_pow(b, mod - 2));
}

void make_fact(ll n)
{
  fac[0] = 1;
  rep(i, n)
  {
    fac[i + 1] = mod_mul(fac[i], i + 1);
  }
}

void make_invfact(ll n)
{
  invfac[n] = mod_pow(fac[n], mod - 2);
  for (int i = n - 1; i >= 0; i--)
  {
    invfac[i] = mod_mul(invfac[i + 1], i + 1);
  }
}