#include <bits/stdc++.h>
using namespace std;

#define INF_LL (int64)1e18
#define INF (int32)1e9
#define REP(i, n) for(int64 i = 0;i < (n);i++)
#define FOR(i, a, b) for(int64 i = (a);i < (b);i++)
#define all(x) x.begin(),x.end()
#define fs first
#define sc second

using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;

const double eps = 1e-10;

template<typename A, typename B>inline void chmin(A &a, B b){if(a > b) a = b;}
template<typename A, typename B>inline void chmax(A &a, B b){if(a < b) a = b;}

template<typename T>
vector<T> make_v(size_t a){return vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
	return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value!=0>::type
fill_v(U &u,const V... v){u=U(v...);}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value==0>::type
fill_v(U &u,const V... v){
	for(auto &e:u) fill_v<T>(e,v...);
}

template<class ValueMonoid, template<class...> class Container=::std::vector>
class SegTree{
public:
  using value_structure = ValueMonoid;
  using value_type = typename value_structure::value_type;
  using const_reference = const value_type &;
  using container_type = Container<value_type>;
  using size_type = typename container_type::size_type;

private:
  ::std::vector<value_type> tree;
  size_type size_;

  static size_type getsize(const size_type x){
    size_type ret = 1;
    while(ret < x)
      ret <<= 1;
    return ret;
  }

  inline value_type calc(const value_type a, const value_type b){
    return value_structure::operation(a, b);
  }

  inline void calc_node(const size_type index){
    if(tree.size() <= (index << 1 | 1)) return;
    tree[index] = value_structure::operation(tree[index<<1], tree[index<<1 | 1]);
  }
public:
  SegTree() : size_(0), tree(){}
  SegTree(const size_type size)
    : size_(size), tree(size << 1, value_structure::identity()){}
  template<class InputIterator>
  SegTree(InputIterator first, InputIterator last)
    : size_(::std::distance(first, last)){
    tree = container_type(size_, value_structure::identity());
    tree.insert(tree.end(), first, last);
    for(size_type i = size_;i > 0;i--){
      calc_node(i);
    }
  }

  size_type size() const { return size_; }
  const_reference operator[](const size_type k) const {
    assert(k < size_);
    return tree[k+size_];
  }

  value_type query(size_type l, size_type r){
    assert(l <= r);
    assert(0 <= l && l < size_);
    assert(0 <= r && r <= size_);
    value_type retl = value_structure::identity(), retr = value_structure::identity();
    for(l += size_, r += size_; l < r ; l >>= 1, r >>= 1){
      if(l&1) retl = calc(retl, tree[l++]);
      if(r&1) retr = calc(tree[--r], retr);
    }
    return calc(retl, retr);
  }

  template<class F>
  void update(size_type index, const F& f){
    assert(0 <= index && index < size());
    index += size_;
    tree[index] = f(::std::move(tree[index]));
    while(index >>= 1)
      calc_node(index);
  }

  /*
  template<class F>
  size_type search(const F& f) const { // [0, result) is True and [0, result-1) is not.
    if(f(value_structure::identity()))
      return 0;
    if(!f(tree[1]))
      return size_+1;
    value_type acc = value_structure::identity();
    size_type i = 1;
    while(i <
  }
  */
};

class v_monoid {
public:
  using value_type = int64;
  static value_type identity() { return INF_LL; }
  static value_type initializer() { return INF_LL; }
  static value_type operation(const value_type& a, const value_type& b) {
    return min(a, b);
  }
};

int main(void){
	cin.tie(0);
	ios::sync_with_stdio(false);

  int64 N, M;
  cin >> N >> M;
  using T = tuple<int64, int64, int64>;
  vector<T> edge(M);
  REP(i, M) {
    int64 a, b, c;
    cin >> a >> b >> c; a--;
    edge[i] = T(a, b, c);
  }
  sort(all(edge));
  SegTree<v_monoid> sg(N+1);
  sg.update(0, [](const int64& a){ return 0; });
  REP(i, M) {
    int64 l, r, c;
    tie(l, r, c) = edge[i];
    sg.update(r-1, [&](const int64& x) { return min(x, sg.query(l, r)+c); });
  }
  int64 res = sg.query(N-1, N);
  if (res == INF_LL) cout << -1 << endl;
  else cout << res << endl;
}


Measured memory_bytes: 4096