#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
using ll = long long int;
const ll MD = 1000000007ll;
const long double PI = 3.1415926535897932384626433832795L;
inline void assert_call(bool assertion, function<void()> f) {
  if (!assertion) {
    cerr << "assertion fault:" << endl;
    f();
    abort();
  }
}
template <typename T1, typename T2>
inline ostream& operator<<(ostream& o, const pair<T1, T2> p) {
  o << '(' << p.first << ':' << p.second << ')';
  return o;
}
template <typename Vec>
inline ostream& _ostream_vecprint(ostream& os, const Vec& a) {
  os << '[';
  for (const auto& e : a) os << ' ' << e << ' ';
  os << ']';
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& o, const vector<T>& v) {
  return _ostream_vecprint(o, v);
}
template <typename T, size_t S>
inline ostream& operator<<(ostream& o, const array<T, S>& v) {
  return _ostream_vecprint(o, v);
}
template <typename T>
inline T& chmax(T& to, const T& val) {
  return to = max(to, val);
}
template <typename T>
inline T& chmin(T& to, const T& val) {
  return to = min(to, val);
}
void bye(string s, int code = 0) {
  cout << s << endl;
  exit(code);
}
mt19937_64 randdev(8901016);
template <typename T, typename Random = decltype(randdev),
          typename enable_if<is_integral<T>::value>::type* = nullptr>
inline T rand(T l, T h, Random& rand = randdev) {
  return uniform_int_distribution<T>(l, h)(rand);
}
template <typename T, typename Random = decltype(randdev),
          typename enable_if<is_floating_point<T>::value>::type* = nullptr>
inline T rand(T l, T h, Random& rand = randdev) {
  return uniform_real_distribution<T>(l, h)(rand);
}
class MaiScanner {
  FILE* fp_;

 public:
  inline MaiScanner(FILE* fp) : fp_(fp) {}
  template <typename T>
  void input_integer(T& var) noexcept {
    var = 0;
    T sign = 1;
    int cc = getc_unlocked(fp_);
    for (; cc < '0' || '9' < cc; cc = getc_unlocked(fp_))
      if (cc == '-') sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = getc_unlocked(fp_))
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
  }
  inline int c() noexcept { return getc_unlocked(fp_); }
  inline MaiScanner& operator>>(int& var) noexcept {
    input_integer<int>(var);
    return *this;
  }
  inline MaiScanner& operator>>(long long& var) noexcept {
    input_integer<long long>(var);
    return *this;
  }
  inline MaiScanner& operator>>(string& var) {
    int cc = getc_unlocked(fp_);
    for (; !(0x21 <= (cc) && (cc) <= 0x7E); cc = getc_unlocked(fp_))
      ;
    for (; (0x21 <= (cc) && (cc) <= 0x7E); cc = getc_unlocked(fp_))
      var.push_back(cc);
    return *this;
  }
  template <typename IT>
  inline void in(IT begin, IT end) {
    for (auto it = begin; it != end; ++it) *this >> *it;
  }
};
class MaiPrinter {
  FILE* fp_;

 public:
  inline MaiPrinter(FILE* fp) : fp_(fp) {}
  template <typename T>
  void output_integer(T var) noexcept {
    if (var == 0) {
      putc_unlocked('0', fp_);
      return;
    }
    if (var < 0) putc_unlocked('-', fp_), var = -var;
    char stack[32];
    int stack_p = 0;
    while (var) stack[stack_p++] = '0' + (var % 10), var /= 10;
    while (stack_p) putc_unlocked(stack[--stack_p], fp_);
  }
  inline MaiPrinter& operator<<(char c) noexcept {
    putc_unlocked(c, fp_);
    return *this;
  }
  inline MaiPrinter& operator<<(int var) noexcept {
    output_integer<int>(var);
    return *this;
  }
  inline MaiPrinter& operator<<(long long var) noexcept {
    output_integer<long long>(var);
    return *this;
  }
  inline MaiPrinter& operator<<(char* str_p) noexcept {
    while (*str_p) putc_unlocked(*(str_p++), fp_);
    return *this;
  }
  inline MaiPrinter& operator<<(const string& str) {
    const char* p = str.c_str();
    const char* l = p + str.size();
    while (p < l) putc_unlocked(*p++, fp_);
    return *this;
  }
  template <typename IT>
  void join(IT begin, IT end, char sep = ' ') {
    for (bool b = 0; begin != end; ++begin, b = 1)
      b ? *this << sep << *begin : *this << *begin;
  }
};
MaiScanner scanner(stdin);
MaiPrinter printer(stdout);
template <typename T>
class SegmentTree {
 public:
  struct IdxVal {
    int index;
    T value;
    inline IdxVal(int _i = 0, const T& _v = T()) : index(_i), value(_v) {}
    inline void set(int i, const T& v) {
      index = i;
      value = v;
    }
    inline bool operator<(const T& t) const { return value < t; }
    inline bool operator==(const T& t) const { return value == t; }
    inline bool operator<(const IdxVal& iv) const {
      return value < iv.value || (value == iv.value && index < iv.index);
    }
    inline bool operator==(const IdxVal& iv) const {
      return index == iv.index && value == iv.value;
    }
  };
  enum struct Cmd { None, Add, Flat };
  struct Node {
    T sum, lazyValue;
    IdxVal max;
    Cmd lazyType;
  };

 private:
  const int size_;
  vector<T> leaf_data_;
  vector<Node> node_data_;
  T* leaf_;
  Node* node_;
  void _resize() {
    leaf_data_.resize(size_);
    node_data_.resize(size_ - 1);
    leaf_ = &leaf_data_[0];
    node_ = &node_data_[0] - 1;
  }
  void _init() {
    for (int i = size_ - 1; 0 < i; --i) {
      int c = i << 1;
      if (size_ <= c)
        node_[i].max.set(c - size_, leaf_[c - size_]);
      else
        node_[i].max = node_[c].max;
      node_[i].lazyType = Cmd::None;
      node_[i].sum = node_[i].lazyValue = 0;
    }
  }

 public:
  inline void fill(T val) {
    std::fill(leaf_data_.begin(), leaf_data_.end(), val);
    _init();
  }
  SegmentTree(int _n)
      : size_([](int _n) {
          int s = 8;
          while (s < _n) s <<= 1;
          return s;
        }(_n)) {
    _resize();
    _init();
  }
  SegmentTree(int _n, const T& _fillVal)
      : size_([](int _n) {
          int s = 8;
          while (s < _n) s <<= 1;
          return s;
        }(_n)) {
    _resize();
    fill(_fillVal);
  }

 private:
  inline void _applyDown(int ptr, int width) {
    width >>= 1;
    if (node_[ptr].lazyType == Cmd::Add) {
      int c = ptr << 1;
      if (size_ <= c) {
        leaf_[c - size_] += node_[ptr].lazyValue;
        leaf_[c + 1 - size_] += node_[ptr].lazyValue;
      } else {
        node_[c].sum += node_[ptr].lazyValue * width;
        node_[c].max.value += node_[ptr].lazyValue;
        node_[c].lazyValue += node_[ptr].lazyValue;
        if (node_[c].lazyType == Cmd::None) node_[c].lazyType = Cmd::Add;
        node_[c + 1].sum += node_[ptr].lazyValue * width;
        node_[c + 1].max.value += node_[ptr].lazyValue;
        node_[c + 1].lazyValue += node_[ptr].lazyValue;
        if (node_[c + 1].lazyType == Cmd::None)
          node_[c + 1].lazyType = Cmd::Add;
      }
      node_[ptr].lazyType = Cmd::None;
      node_[ptr].lazyValue = 0;
    } else if (node_[ptr].lazyType == Cmd::Flat) {
      node_[ptr].lazyType = Cmd::None;
      int c = ptr << 1;
      if (size_ <= c) {
        chmax(leaf_[c - size_], node_[ptr].lazyValue);
        chmax(leaf_[c + 1 - size_], node_[ptr].lazyValue);
      } else {
        node_[c].sum = node_[ptr].lazyValue * width;
        chmax(node_[c].max.value, node_[ptr].lazyValue);
        node_[c].lazyValue = node_[ptr].lazyValue;
        node_[c].lazyType = Cmd::Flat;
        node_[c + 1].sum = node_[ptr].lazyValue * width;
        chmax(node_[c + 1].max.value, node_[ptr].lazyValue);
        node_[c + 1].lazyValue = node_[ptr].lazyValue;
        node_[c + 1].lazyType = Cmd::Flat;
      }
      node_[ptr].lazyType = Cmd::None;
      node_[ptr].lazyValue = 0;
    }
  }
  inline void _applyUp(int ptr) {
    if (size_ <= ptr << 1) {
      int i = (ptr << 1) - size_;
      if (leaf_[i] < leaf_[i + 1])
        node_[ptr].max.set(i + 1, leaf_[i + 1]);
      else
        node_[ptr].max.set(i, leaf_[i]);
      node_[ptr].sum = leaf_[i] + leaf_[i + 1];
    } else {
      if (node_[(ptr << 1)].max.value < node_[(ptr << 1) + 1].max.value)
        node_[ptr].max = node_[(ptr << 1) + 1].max;
      else
        node_[ptr].max = node_[(ptr << 1)].max;
      node_[ptr].sum = node_[ptr << 1].sum + node_[(ptr << 1) + 1].sum;
    }
  }
  void _downward_applyDown(int ptr1, int width = 1) {
    if (1 < ptr1) _downward_applyDown(ptr1 >> 1, width << 1);
    _applyDown(ptr1, width);
  }
  void _setValueRange(int begin, int end, T val, int ptr, int rangebegin,
                      int rangeend) {
    if (rangeend <= begin || end <= rangebegin) return;
    if (begin <= rangebegin && rangeend <= end) {
      if (size_ <= ptr) {
        chmax(leaf_[rangebegin], val);
      } else {
        node_[ptr].sum = val * (rangeend - rangebegin);
        chmax(node_[ptr].max.value, val);
        node_[ptr].lazyType = Cmd::Flat;
        node_[ptr].lazyValue = val;
        _applyDown(ptr, rangeend - rangebegin);
      }
      return;
    }
    _applyDown(ptr, rangeend - rangebegin);
    _setValueRange(begin, end, val, ptr << 1, rangebegin,
                   (rangebegin + rangeend) >> 1);
    _setValueRange(begin, end, val, (ptr << 1) + 1,
                   (rangebegin + rangeend) >> 1, rangeend);
    _applyUp(ptr);
  }
  void _addValueRange(int begin, int end, T val, int ptr, int rangebegin,
                      int rangeend) {
    if (rangeend <= begin || end <= rangebegin) return;
    if (begin <= rangebegin && rangeend <= end) {
      if (size_ <= ptr) {
        leaf_[rangebegin] += val;
      } else {
        node_[ptr].sum += val * (rangeend - rangebegin);
        node_[ptr].max.value += val;
        node_[ptr].lazyValue += val;
        if (node_[ptr].lazyType == Cmd::None) node_[ptr].lazyType = Cmd::Add;
      }
      return;
    }
    _applyDown(ptr, rangeend - rangebegin);
    _addValueRange(begin, end, val, ptr << 1, rangebegin,
                   (rangebegin + rangeend) >> 1);
    _addValueRange(begin, end, val, (ptr << 1) + 1,
                   (rangebegin + rangeend) >> 1, rangeend);
    _applyUp(ptr);
  }
  T _getSumRange(int begin, int end, int ptr, int rangebegin, int rangeend) {
    if (rangeend <= begin || end <= rangebegin) return 0;
    if (begin <= rangebegin && rangeend <= end) {
      if (size_ <= ptr)
        return leaf_[rangebegin];
      else
        return node_[ptr].sum;
    }
    _applyDown(ptr, rangeend - rangebegin);
    return _getSumRange(begin, end, ptr << 1, rangebegin,
                        (rangebegin + rangeend) >> 1) +
           _getSumRange(begin, end, (ptr << 1) + 1,
                        (rangebegin + rangeend) >> 1, rangeend);
  }
  IdxVal _getMaxRange(int begin, int end, int ptr, int rangebegin,
                      int rangeend) {
    if (rangeend <= begin || end <= rangebegin) return IdxVal(-1);
    if (begin <= rangebegin && rangeend <= end) {
      if (size_ <= ptr)
        return IdxVal(rangebegin, leaf_[rangebegin]);
      else {
        _applyDown(ptr, rangeend - rangebegin);
        return node_[ptr].max;
      }
    }
    _applyDown(ptr, rangeend - rangebegin);
    auto l = _getMaxRange(begin, end, ptr << 1, rangebegin,
                          (rangebegin + rangeend) >> 1);
    auto r = _getMaxRange(begin, end, (ptr << 1) + 1,
                          (rangebegin + rangeend) >> 1, rangeend);
    if (l.index == -1) return move(r);
    if (r.index == -1) return move(l);
    return l.value < r.value ? move(r) : move(l);
  }

 public:
  inline T getValue(int index) {
    _downward_applyDown((index + size_) >> 1);
    return leaf_[index];
  }
  inline void setValue(int index, T val) {
    _downward_applyDown((index + size_) >> 1);
    T diff = val - leaf_[index];
    leaf_[index] = val;
    for (int ptr = (index + size_); 1 < ptr;) {
      ptr >>= 1;
      node_[ptr].sum -= diff;
      _applyUp(ptr);
    }
  }
  inline void addValue(int index, T val) {
    _downward_applyDown((index + size_) >> 1);
    leaf_[index] += val;
    for (int ptr = (index + size_); 1 < ptr;) {
      ptr >>= 1;
      node_[ptr].sum += val;
      _applyUp(ptr);
    }
  }
  inline void setValueRange(int begin, int end, T val) {
    _setValueRange(begin, end, val, 1, 0, size_);
  }
  inline void addValueRange(int begin, int end, T val) {
    _addValueRange(begin, end, val, 1, 0, size_);
  }
  inline T getSumRange(int begin, int end) {
    return _getSumRange(begin, end, 1, 0, size_);
  }
  inline IdxVal getMaxRange(int begin, int end) {
    return _getMaxRange(begin, end, 1, 0, size_);
  }
};
int N, M;
array<int, 3> edges[100010];
const ll inf = 1e18;
int main() {
  scanner >> N >> M;
  SegmentTree<ll> dist(N);
  dist.fill(-inf);
  dist.setValue(0, 0);
  for (auto i = remove_reference<decltype(M)>::type(); (i) < (M); ++(i)) {
    int l, r, c;
    scanner >> l >> r >> c;
    edges[i] = {l - 1, r, c};
  }
  sort(edges, edges + M);
  for (auto i = remove_reference<decltype(M)>::type(); (i) < (M); ++(i)) {
    int l, r, c;
    l = edges[i][0];
    r = edges[i][1];
    c = edges[i][2];
    auto mi = dist.getMaxRange(l, r);
    ll v = -mi.value;
    dist.setValueRange(l, r, -(v + c));
  }
  ll ans = -dist.getValue(N - 1);
  if (ans >= inf) bye("-1");
  cout << ans << endl;
  return 0;
}
