#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define REPR(i,n) for (int i=(int)(n)-1;i>=0;--i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define valid(y,x,h,w) (0<=y&&y<h&&0<=x&&x<w)
#define tpl(...) make_tuple(__VA_ARGS__)
const int INF = 0x3f3f3f3f;
const double EPS = 1e-8;
const double PI = acos(-1);
const int dy[] = {-1,0,1,0};
const int dx[] = {0,1,0,-1};
typedef long long ll;
typedef pair<int,int> pii;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename Ch,typename Tr,typename C,typename=decltype(begin(C()))>basic_ostream<Ch,Tr>& operator<<(basic_ostream<Ch,Tr>&os,
const C& c){os<<'[';for(auto i=begin(c);i!=end(c);++i)os<<(i==begin(c)?"":" ")<<*i;return os<<']';}
template<class S,class T>ostream&operator<<(ostream &o,const pair<S,T>&t){return o<<'('<<t.first<<','<<t.second<<')';}
template<int N,class Tp>void output(ostream&,const Tp&){}
template<int N,class Tp,class,class ...Ts>void output(ostream &o,const Tp&t){if(N)o<<',';o<<get<N>(t);output<N+1,Tp,Ts...>(o,t);}
template<class ...Ts>ostream&operator<<(ostream&o,const tuple<Ts...>&t){o<<'(';output<0,tuple<Ts...>,Ts...>(o,t);return o<<')';}
template<class T>void output(T t,char z=10){if(t<0)t=-t,putchar(45);int c[20];
int k=0;while(t)c[k++]=t%10,t/=10;for(k||(c[k++]=0);k;)putchar(c[--k]^48);putchar(z);}
template<class T>void outputs(T t){output(t);}
template<class S,class ...T>void outputs(S a,T...t){output(a,32);outputs(t...);}
template<class T>void output(T *a,int n){REP(i,n)cout<<a[i]<<(i!=n-1?',':'\n');}
template<class T>void output(T *a,int n,int m){REP(i,n)output(a[i],m);}
template<class T>bool input(T &t){int n=1,c;for(t=0;!isdigit(c=getchar())&&~c&&c-45;);
if(!~c)return 0;for(c-45&&(n=0,t=c^48);isdigit(c=getchar());)t=10*t+c-48;t=n?-t:t;return 1;}
template<class S,class ...T>bool input(S&a,T&...t){input(a);return input(t...);}
template<class T>bool inputs(T *a, int n) { REP(i,n) if(!input(a[i])) return 0; return 1;}

template<class F> struct Data {
  typedef typename F::T T;
  T x;
  Data() : x(F::e) {}
  Data(T x) : x(x) {}
  Data(const Data &a, const Data &b) { x = F::f(a.x,b.x); }
};

template<typename _T> struct MIN {
  typedef _T T;
  static constexpr T e = 1LL<<60;
  static T f(const T &a, const T &b) { return min(a,b); }
};

template<class Data, int N>
struct SegTree {
  typedef typename Data::T T;
  typedef Data D;
  int m;
  Data dat[N*4];
  void init(int n) { for (m=1; m<n; m*=2); REP(i,2*m-1) dat[i] = Data(); }
  void setleaf(int idx, const Data &d) { dat[m-1+idx] = d; }
  void finishset() { for (int i=m-2; i>=0; --i) dat[i] = Data(dat[i*2+1],dat[i*2+2]); }
  void init(T *v, int n) { init(n); REP(i,n) setleaf(i,v[i]); finishset(); }
  void change(int k, const Data &d) {
    k += m-1;
    dat[k] = d;
    while(k) {
      k = (k-1)>>1;
      dat[k] = Data(dat[(k<<1)+1],dat[(k<<1)+2]);
    }
  }
  Data query(int a, int b) { return q(a, b, 0, 0, m); }
  Data q(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l) return Data();
    if (a <= l && r <= b) return dat[k];
    const Data &ld = q(a, b, k*2+1, l, (l+r)/2);
    const Data &rd = q(a, b, k*2+2, (l+r)/2, r);
    return Data(ld,rd);
  }
};

const int MAX_N = 100000;

typedef SegTree<Data<MIN<ll> >, MAX_N> RMQ;

struct P {
  int l,r,c;
  bool operator<(const P &rhs) const {
    return r <rhs.r;
  }
};

P p[100000];
RMQ rmq;

int main() {
  int n, m;
  while(input(n,m)) {
    REP(i,m) {
      input(p[i].l,p[i].r,p[i].c);
      p[i].l--;
      p[i].r--;
    }
    sort(p,p+m);
    rmq.init(n);
    rmq.change(0,0);
    REP(i,m) {
      auto c = rmq.query(p[i].l, p[i].r).x + p[i].c;
      auto v = rmq.query(p[i].r, p[i].r+1).x;
      if (v > c) rmq.change(p[i].r,c);
    }
    ll v = rmq.query(n-1,n).x;
    if (v == (1LL<<60)) cout << -1 << endl;
    else cout << v << endl;
  }
}


Measured memory_bytes: 4480