#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll INF = LLONG_MAX / 40 - 100;
ll mod = 1e9 + 7;
long long dx[] = {-1, 0, 1, 0}, dy[] = {0, 1, 0, -1};
long long ddx[] = {-1, 0, 1, -1, 1, -1, 0, 1},
          ddy[] = {1, 1, 1, 0, 0, -1, -1, -1};
vector<ll> prime;
class fact {
 public:
  long long fmod = 1e9 + 7;
  vector<long long> fac, inv;
  fact(long long n, long long Mod = 1e9 + 7) {
    fmod = Mod;
    fac = vector<long long>(n + 1, 0);
    inv = vector<long long>(n + 1, 0);
    fac[0] = 1;
    for (long long i = 1; i < n + 1; i++) fac[i] = fac[i - 1] * i % fmod;
    for (long long i = 0; i < n + 1; i++) inv[i] = fact::POW(fac[i], fmod - 2);
  }
  ll nCr(ll n, ll r) {
    if (n < r) return 0;
    return (fac[n] * inv[r] % fmod) * inv[n - r] % fmod;
  }
  ll POW(ll a, ll b) {
    ll c = 1;
    while (b > 0) {
      if (b & 1) {
        c = a * c % fmod;
      }
      a = a * a % fmod;
      b >>= 1;
    }
    return c;
  }
};
template <class T = ll>
T in() {
  T x;
  cin >> x;
  return (x);
}
void DEBUG(vector<long long> a) {
  for (long long i = 0; i < a.size(); i++) cout << a[i] << " ";
  cout << endl;
}
void EMP(long long x) { cout << "!!!" << x << "!!!" << endl; }
ll GCD(ll a, ll b) {
  ll c;
  while (b != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
ll LCM(ll a, ll b) { return (a / GCD(a, b)) * b; }
ll POW(ll a, ll b) {
  ll c = 1;
  while (b > 0) {
    if (b & 1) {
      c = a * c % mod;
    }
    a = a * a % mod;
    b >>= 1;
  }
  return c;
}
void PRI(ll n) {
  bool a[n + 1LL];
  for (long long i = 0; i < n + 1LL; i++) {
    a[i] = 1LL;
  }
  for (long long i = 2; i < n + 1LL; i++) {
    if (a[i]) {
      prime.push_back(i);
      ll b = i;
      while (b <= n) {
        a[b] = 0;
        b += i;
      }
    }
  }
}
template <typename T>
T chmin(T& a, T b) {
  if (a > b) a = b;
  return a;
}
template <typename T>
T chmax(T& a, T b) {
  if (a < b) a = b;
  return b;
}
bool isSqrt(ll a) { return pow(sqrt(a), 2) == a ? 1 : 0; }
void YesNo(bool a) {
  if (a)
    cout << "Yes";
  else
    cout << "No";
  cout << endl;
}
void yesno(bool a) {
  if (a)
    cout << "yes";
  else
    cout << "no";
  cout << endl;
}
void YESNO(bool a) {
  if (a)
    cout << "YES";
  else
    cout << "NO";
  cout << endl;
}
long long extGCD(long long a, long long b, long long& x, long long& y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  long long d = extGCD(b, a % b, y, x);
  y -= a / b * x;
  return d;
}
long long mul(long long a, long long b, long long mod) {
  long long keta[20] = {};
  long long c = b;
  long long now = 0;
  while (c != 0ll) {
    keta[now] = c % 10;
    c /= 10;
    now++;
  }
  long long res = 0;
  for (long long i = 19; i >= 0; --i) {
    res *= 10;
    res %= mod;
    res += keta[i] * a % mod;
  }
  return res % mod;
}
bool cmp(pair<pair<long long, long long>, long long> a,
         pair<pair<long long, long long>, long long> b) {
  if (a.first.first != b.first.first) return a.first.first < b.first.first;
  if (a.first.second != b.first.second) return a.first.second < b.first.second;
  return a.second < b.second;
}
vector<pair<long long, long long>> g[100005];
void solve() {
  long long n, m;
  cin >> n >> m;
  vector<pair<pair<long long, long long>, long long>> lrc(m);
  for (long long(i) = (0); (i) < (m); (i)++) {
    long long l, r, c;
    cin >> l >> r >> c;
    --l, --r;
    lrc[i] = {{l, r}, c};
  }
  sort((lrc).begin(), (lrc).end(), cmp);
  set<long long> cnd{0};
  long long now = 0;
  vector<long long> cost(n, INF);
  cost[0] = 0;
  for (long long(i) = (0); (i) < (m); (i)++) {
    long long l, r, c;
    l = lrc[i].first.first;
    r = lrc[i].first.second;
    c = lrc[i].second;
    set<long long> tmp = cnd;
    for (auto nxt : cnd) {
      if (nxt < l)
        tmp.erase(nxt);
      else if (nxt < r) {
        tmp.insert(r);
        g[nxt].push_back({r, c});
      } else {
        break;
      }
    }
    cnd = tmp;
  }
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      pq;
  pq.push({0, 0});
  while (!pq.empty()) {
    long long nxt = pq.top().second, cst = pq.top().first;
    pq.pop();
    if (cost[nxt] < cst) continue;
    for (auto to : g[nxt]) {
      if (cost[to.first] > cst + to.second) {
        cost[to.first] = cst + to.second;
        pq.push({cost[to.first], to.first});
      }
    }
  }
  if (cost[n - 1] == INF)
    cout << -1 << endl;
  else
    cout << cost[n - 1] << endl;
  return;
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}
