#include <bits/stdc++.h>
#define whlie while
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define rep(i,N) for(int i = 0; i < (N); i++)
#define repr(i,N) for(int i = (N) - 1; i >= 0; i--)
#define rep1(i,N) for(int i = 1; i <= (N) ; i++)
#define repr1(i,N) for(int i = (N) ; i > 0 ; i--)
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define vrep(v,it) for(auto it = v.begin(); it != v.end(); it++)
#define vrepr(v,it) for(auto it = v.rbegin(); it != v.rend(); it++)
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) ll __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
using namespace std; void solve();
using ll = long long; using vl = vector<ll>;
using vi = vector<int>; using vvi = vector< vector<int> >;
constexpr int inf = 1001001001;
constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); rep(i,s) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}
#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);cout << endl;} while(0)
#else
  #define trc(...)
  #define trca(...)
  int main(){solve();}
#endif
using P = pair<int,int>; using vp = vector<P>;
constexpr int MOD = /** 1000000007; //*/ 998244353;
////////////////

// 遅延伝播セグメント木
template <typename T,typename E,typename F,typename G,typename H>
struct LST{
  int n,height;
  F f;
  G g;
  H h;
  T ti;
  E ei;
  vector<T> dat;
  vector<E> laz;
  LST(int n, F f,G g,H h,T ti,E ei):
    f(f),g(g),h(h),ti(ti),ei(ei){
      init(n);
  }
  LST(const vector<T> &v, F f,G g,H h,T ti,E ei):
    f(f),g(g),h(h),ti(ti),ei(ei){
      init( (int)v.size() );
      build(v);
  }
  
  void init(int n_){
    n=1;height=0;
    while(n<n_) n<<=1,height++;
    dat.assign(2*n,ti);
    laz.assign(2*n,ei);
  }
  void build(const vector<T> &v){
    int n_=v.size();
    init(n_);
    for(int i=0;i<n_;i++) dat[n+i]=v[i];
    for(int i=n-1;i;i--)
      dat[i]=f(dat[(i<<1)|0],dat[(i<<1)|1]);
  }
  inline T reflect(int k){
    return laz[k]==ei?dat[k]:g(dat[k],laz[k]);
  }
  inline void eval(int k){
    if(laz[k]==ei) return;
    laz[(k<<1)|0]=h(laz[(k<<1)|0],laz[k]);
    laz[(k<<1)|1]=h(laz[(k<<1)|1],laz[k]);
    dat[k]=reflect(k);
    laz[k]=ei;
  }
  inline void thrust(int k){
    for(int i=height;i;i--) eval(k>>i);
  }
  inline void recalc(int k){    
    while(k>>=1)
      dat[k]=f(reflect((k<<1)|0),reflect((k<<1)|1));
  }
  void update(int a,int b,E x){
    thrust(a+=n);
    thrust(b+=n-1);
    for(int l=a,r=b+1;l<r;l>>=1,r>>=1){
      if(l&1) laz[l]=h(laz[l],x),l++;
      if(r&1) --r,laz[r]=h(laz[r],x);
    }
    recalc(a);
    recalc(b);
  }
  void set_val(int a,T x){
    thrust(a+=n);
    dat[a]=x;laz[a]=ei;
    recalc(a);
  }
  T query(int a,int b){
    thrust(a+=n);
    thrust(b+=n-1);
    T vl=ti,vr=ti;
    for(int l=a,r=b+1;l<r;l>>=1,r>>=1) {
      if(l&1) vl=f(vl,reflect(l++));
      if(r&1) vr=f(reflect(--r),vr);
    }
    return f(vl,vr);
  }
};

void solve(){
  ini(N , M);
  vi L(M) , R(M); vl C(M);
  rep(i , M) {
    in(L[i] , R[i] , C[i]);
    L[i]--; R[i]--;
  }
  vi ord(M); iota(all(ord) , 0);
  sort(all(ord),[&](int l,int r){
    //return R[l] == R[r] ? L[l] < L[r] : R[l] < R[r];
    return L[l] < L[r];
  });
  int oi = 0;

  // 区間をxに更新・区間最小を取得 
  // (区間更新一点取得にも使える)
  constexpr ll T_UNIT = infLL;
  constexpr ll E_UNIT = T_UNIT;
  vector<ll> v(N, T_UNIT);
  auto f=[](ll a, ll b){
    return min(a , b);
  };
  auto gg=[](ll a, ll b){
    return min(a , b);
  };
  auto h=[](ll a, ll b){
    return min(a , b);
  };
  LST<ll,ll,decltype(f),decltype(gg),decltype(h)>
    lst(v,f,gg,h,T_UNIT,E_UNIT);

  lst.update(0 , 1 , 0);
  rep1(i , N - 1){
    while(oi != M && L[ord[oi]] < i){
      // 更新
      ll nya = lst.query(L[ord[oi]] , L[ord[oi]] + 1);
      lst.update( L[ord[oi]] + 1, R[ord[oi]] + 1 , nya + C[ord[oi]]);
      oi++;
    }
  }
  ll ans = lst.query(N - 1 , N);
  out(ans >= infLL ? -1 : ans);
}


Measured memory_bytes: 7040