#include <bits/stdc++.h>
using namespace std;
inline long long modpow(long long x, long long y,
                        long long m = (long long)((1e9) + 7)) {
  long long res = 1;
  while (y) {
    if (y % 2) {
      res *= x;
      res %= m;
    }
    x = x * x % m;
    y /= 2;
  }
  return res;
}
long long mypow(long long x, long long y) {
  long long res = 1;
  while (y) {
    if (y % 2) {
      res *= x;
    }
    x = x * x;
    y /= 2;
  }
  return res;
}
bool prime(long long x) {
  for (long long i = 2; i * i <= x; i++) {
    if (!(x % i)) {
      return false;
    }
  }
  return true;
}
inline long long gcd(long long x, long long y) {
  if (!y) {
    return x;
  }
  return gcd(y, x % y);
}
long long keta(long long x) {
  long long ans = 0;
  while (x) {
    x /= 10;
    ans++;
  }
  return ans;
}
long long ketasum(long long x) {
  long long ans = 0;
  while (x) {
    ans += x % 10;
    x /= 10;
  }
  return ans;
}
inline long long lcm(long long x, long long y) {
  long long ans = x / gcd(x, y) * y;
  return ans;
}
long long twobeki(long long x) {
  long long ans = 0;
  while (1) {
    if (!(x & 1)) {
      ans++;
      x /= 2;
    } else {
      break;
    }
  }
  return ans;
}
template <class T, class U>
inline bool chmax(T& lhs, const U& rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U>
inline bool chmin(T& lhs, const U& rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
void Yes() { cout << "Yes" << endl; }
void No() { cout << "No" << endl; }
void YES() { cout << "YES" << endl; }
void NO() { cout << "NO" << endl; }
class modInt {
  long long value, modulo;

 public:
  modInt() : value(0), modulo((long long)((1e9) + 7)) { value = 0; }
  template <typename T>
  modInt(T value = 0, long long modulo = (long long)((1e9) + 7))
      : value(value), modulo(modulo) {
    if (value < 0) {
      value = -value;
      value %= modulo;
      value = -value;
      value += modulo;
    }
    this->value = value % modulo;
  }
  inline operator long long() const { return value; }
  inline modInt& operator+=(modInt x) {
    value += x.value;
    if (value >= modulo) value -= modulo;
    return *this;
  }
  inline modInt& operator++() {
    if (value == modulo - 1)
      value = 0;
    else
      value++;
    return *this;
  }
  inline modInt& operator-() const { return modInt(0) -= *this; }
  inline modInt& operator-=(modInt x) {
    value -= x.value;
    if (value < 0) value += modulo;
    return *this;
  }
  inline modInt& operator--() {
    if (value == 0)
      value = modulo - 1;
    else
      value--;
    return *this;
  }
  inline modInt& operator*=(modInt x) {
    value = value * x.value % modulo;
    return *this;
  }
  inline modInt& operator/=(modInt x) { return operator*=(x.inv()); }
  inline modInt inv() { return modpow(*this, modulo - 2); }
  template <typename T>
  modInt operator+(T x) {
    return modInt(*this) += x;
  }
  template <typename T>
  modInt& operator+=(T x) {
    return operator+=(modInt(x, modulo));
  }
  template <typename T>
  modInt operator-(T x) {
    return modInt(*this) -= x;
  }
  template <typename T>
  modInt& operator-=(T x) {
    return operator-=(modInt(x, modulo));
  }
  template <typename T>
  modInt operator*(T x) {
    return modInt(*this) *= x;
  }
  template <typename T>
  modInt& operator*=(T x) {
    return operator*=(modInt(x, modulo));
  }
  template <typename T>
  modInt operator/(T x) {
    return modInt(*this) /= x;
  }
  template <typename T>
  modInt& operator/=(T x) {
    return operator/=(modInt(x, modulo));
  }
};
istream& operator>>(istream& ist, modInt& x) {
  long long a;
  ist >> a;
  x = a;
  return ist;
}
template <class T>
class SegTree {
  long long n;
  vector<T> data;
  T def;
  function<T(T, T)> operation;
  function<T(T, T)> update;
  T _query(long long a, long long b, long long k, long long l, long long r) {
    if (r <= a || b <= l) return def;
    if (a <= l && r <= b)
      return data[k];
    else {
      T c1 = _query(a, b, 2 * k + 1, l, (l + r) / 2);
      T c2 = _query(a, b, 2 * k + 2, (l + r) / 2, r);
      return operation(c1, c2);
    }
  }

 public:
  SegTree(size_t _n, T _def, function<T(T, T)> _operation,
          function<T(T, T)> _update)
      : def(_def), operation(_operation), update(_update) {
    n = 1;
    while (n < _n) {
      n *= 2;
    }
    data = vector<T>(2 * n - 1, def);
  }
  void change(long long i, T x) {
    i += n - 1;
    data[i] = update(data[i], x);
    while (i > 0) {
      i = (i - 1) / 2;
      data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
    }
  }
  T query(long long a, long long b) { return _query(a, b, 0, 0, n); }
  T operator[](long long i) { return data[i + n - 1]; }
};
class Union_Find {
  vector<long long> par;
  vector<long long> rankmy;
  vector<long long> ookisa;

 public:
  Union_Find(long long size) {
    par = vector<long long>(size);
    rankmy = vector<long long>(size);
    ookisa = vector<long long>(size);
    for (long long i = 0; i < size; i++) {
      par[i] = i;
      ookisa[i] = 1;
    }
  }
  long long find(long long x) {
    if (par[x] == x) {
      return x;
    }
    return par[x] = find(par[x]);
  }
  void unite(long long x, long long y) {
    x = find(x);
    y = find(y);
    if (x == y) {
      return;
    }
    if (rankmy[x] < rankmy[y]) {
      par[x] = y;
      ookisa[y] += ookisa[x];
      ookisa[x] = 0;
    } else {
      par[y] = x;
      ookisa[x] += ookisa[y];
      ookisa[y] = 0;
      if (rankmy[x] == rankmy[y]) {
        rankmy[x]++;
      }
    }
  }
  long long size(long long i) {
    i = find(i);
    return ookisa[i];
  }
  bool same(long long x, long long y) { return find(x) == find(y); }
};
long long n, m, l[100004], r[100004], c[100004], kyo[100004];
vector<pair<long long, long long>> h[100004];
signed main() {
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cin >> n >> m;
  for (long long i = 0; i < m; i++) {
    cin >> l[i] >> r[i] >> c[i];
    h[l[i]].push_back({r[i], c[i]});
  }
  fill(kyo, kyo + n + 1, (long long)(3e18));
  kyo[1] = 0;
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      p;
  for (long long i = 1; i <= n; i++) {
    if (p.size()) {
      if (p.top().second < i) {
        p.pop();
      }
    }
    if (p.size()) chmin(kyo[i], p.top().first);
    for (pair<long long, long long> j : h[i]) {
      p.push({kyo[i] + j.second, j.first});
    }
  }
  if (kyo[n] >= (long long)(3e18)) {
    cout << -1 << "\n";
  } else {
    cout << kyo[n] << "\n";
  }
}
