#ifdef DEBUG_IS_VALID
#define DEB 1 
#define _LIBCPP_DEBUG 0
#else
#define DEB 0
#define NDEBUG
#endif

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

#define ALL(g) (g).begin(),(g).end()
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define RREP(i, x, n) for(int i = x; i >= n; i--)
#define rrep(i, n) RREP(i,n,0)
#define pb push_back
#define fi first
#define se second
#pragma GCC optimize ("-O3")

using namespace std;

#define DUMPOUT cout
#define dump(...) if(DEB) DUMPOUT<<"  "<<#__VA_ARGS__<<" :["<<__LINE__<<":"<<__FUNCTION__<<"]"<<endl<<"    "; if(DEB) dump_func(__VA_ARGS__)
template<typename T1,typename T2>ostream& operator << (ostream& os, pair<T1,T2> p){cout << "(" << p.first << ", " << p.second << ")"; return os;}
template<typename T>ostream& operator << (ostream& os, vector<T>& vec) { os << "{"; for (int i = 0; i<vec.size(); i++) os << vec[i] << (i + 1 == vec.size() ? "" : ", "); os << "}"; return os; }
template<typename T>ostream& operator << (ostream& os, set<T>& st){cout << "{"; for(auto itr = st.begin(); itr != st.end(); itr++) cout << *itr << (next(itr)!=st.end() ? ", " : ""); cout << "}"; return os;}
template<typename T1,typename T2>ostream& operator << (ostream& os, map<T1,T2> mp){cout << "{"; for(auto itr = mp.begin(); itr != mp.end(); itr++) cout << "(" << (itr->first) << ", " << (itr->second) << ")" << (next(itr)!=mp.end() ? "," : ""); cout << "}"; return os; }

void dump_func(){DUMPOUT << endl;}
template <class Head, class... Tail>void dump_func(Head&& head, Tail&&... tail){ DUMPOUT << head; if (sizeof...(Tail) == 0) { DUMPOUT << " "; } else { DUMPOUT << ", "; } dump_func(std::move(tail)...);}
template<class T> inline bool chmax(T& a,T const& b){if(a>=b) return false; a=b; return true;}
template<class T> inline bool chmin(T& a,T const& b){if(a<=b) return false; a=b; return true;}

using ll = long long;
using P = pair<int,int>;
using Pl = pair<ll,ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vp = vector<Pl>;
using vvp = vector<vp>;

const int INF = 1<<29;
const long long LINF=1LL<<59;

template <class Monoid> class SegmentTree{
  private:
    using T = typename Monoid::type;
    int sz, N;
    vector<T> dat;
  public:
    SegmentTree(const vector<T> &vec){
      sz = vec.size(); N = 1; while(N<sz) N<<=1;
      dat.resize(2*N);
      fill(dat.begin(),dat.end(),Monoid::id());
      copy(vec.begin(),vec.end(),dat.begin()+N-1);
      for(int i=N-2;i>=0;i--) dat[i]=Monoid::op(dat[2*i+1],dat[2*i+2]);
    }
    SegmentTree(const int n){
      sz = n; N = 1; while(N<n) N<<=1;
      dat.resize(2*N);
      fill(dat.begin(),dat.end(),Monoid::id());
      for(int i=N-2;i>=0;i--) dat[i]=Monoid::op(dat[2*i+1],dat[2*i+2]);
    }
    void update(int i,T val){
      assert(0<=i && i<sz);
      dat[i+=N-1] = val;
      while(i>0){
        i=(i-1)/2;
        dat[i]=Monoid::op(dat[2*i+1],dat[2*i+2]);
      }
      return;
    }
    T query(int a,int b){ // op[a,b)
      return query(a,b,0,0,N);}
    T query(int a,int b,int k,int l,int r){
      assert(0<=l && l<=r && r<2*N);
      if(r<=a || b<=l) return Monoid::id();
      else if(a<=l && r<=b) return dat[k];
      else{
        T lval=query(a,b,2*k+1,l,(l+r)/2);
        T rval=query(a,b,2*k+2,(l+r)/2,r);
        return Monoid::op(lval,rval);
      }
    }
};

struct RMQ{
  using type = ll;
  static type id(){return LINF;}
  static type op(const type &a ,const type &b){return min(a,b);}
};

void solve(long long N, long long M, std::vector<long long> L, std::vector<long long> R, std::vector<long long> C){
  vl idx(M);
  iota(ALL(idx),0LL);
  sort(ALL(idx), [&](ll lhs,ll rhs){return L[lhs]!=L[rhs] ? L[lhs]<L[rhs] : R[lhs]<R[rhs];});
  SegmentTree<RMQ> rmq(N+5);
  rmq.update(1,0LL);
  for(auto i:idx){
    ll mi = rmq.query(L[i],R[i]+1);
    ll val = rmq.query(R[i],R[i]+1);
    rmq.update(R[i],min(val,mi+C[i]));
    // dump(L[i], R[i], val, mi+C[i]);
  }
  ll ans = rmq.query(N,N+1);
  if(ans==LINF) cout << -1 << endl;
  else cout << ans << endl;
}

int main(){
  std::cout << std::fixed << std::setprecision(15);
  long long N;
  scanf("%lld",&N);
  long long M;
  scanf("%lld",&M);
  std::vector<long long> L(M);
  std::vector<long long> R(M);
  std::vector<long long> C(M);
  for(int i = 0 ; i < M ; i++){
    scanf("%lld",&L[i]);
    scanf("%lld",&R[i]);
    scanf("%lld",&C[i]);
  }
  solve(N, M, std::move(L), std::move(R), std::move(C));
  return 0;
}


Measured memory_bytes: 5376