// template version 1.13
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define infile "../test/sample-1.in"
#define int long long //{{{
const int INF=1e18;
const int MOD=1e9+7; //}}}

// define basic macro {{{
#define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define _rrep(i,n) rrepi(i,0,n)
#define rrepi(i,a,b) for(int i=(int)((b)-1);i>=(int)(a);--i)
#define rrep(...) _overload3(__VA_ARGS__,rrepi,_rrep,)(__VA_ARGS__)
#define each(i,a) for (auto&& i : a)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(a, b, c) make_tuple(a, b, c)
#define ceil(a,b) ((a)+(b)-1)/(b)
#define is_in(x, a, b) ((a)<=(x) && (x)<(b))
#define uni(x) sort(all(x));x.erase(unique(all(x)),x.end())
#define ub upper_bound
#define lb lower_bound
#define posl(A, x) (lower_bound(all(A), x)-A.begin())
#define posu(A, x) (upper_bound(all(A),x)-A.begin())
template<class T> inline void chmax(T &a, const T &b) { if((a) < (b)) (a) = (b); }
template<class T> inline void chmin(T &a, const T &b) { if((a) > (b)) (a) = (b); }

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int,int> pii;
typedef tuple<int,int,int> iii;

template<typename T> using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast { Fast(){ std::cin.tie(0); ios::sync_with_stdio(false); } } fast;

#if defined(PCM) || defined(LOCAL)
  #include "lib/dump.hpp"
#else
  #define dump(...) 42
  #define dump_1d(...) 42
  #define dump_2d(...) 42
#endif
//}}}

struct SegmentTree {
  private:
    int n;
    vector<int> node;

  public:
    SegmentTree(vector<int> a) {
      int sz = a.size();
      n = 1; while(n < sz) n *= 2;
      node.resize(2*n-1, INF);
      for(int i=0; i<sz; i++) node[i+n-1] = a[i];
      for(int i=n-2; i>=0; i--) node[i] = min(node[2*i+1], node[2*i+2]);
    }
    SegmentTree(int V) : SegmentTree(vector<int>(V, INF)) {}

    void update(int x, int val) {
      x += (n - 1);
      node[x] = val;
      while(x > 0) {
        x = (x - 1) / 2;
        node[x] = min(node[2*x+1], node[2*x+2]);
      }
    }

    // get min of [l, r)
    int getmin(int a, int b, int k=0, int l=0, int r=-1) {
      if(r < 0) r = n;
      if(r <= a || b <= l) return INF;
      if(a <= l && r <= b) return node[k];

      int vl = getmin(a, b, 2*k+1, l, (l+r)/2);
      int vr = getmin(a, b, 2*k+2, (l+r)/2, r);
      return min(vl, vr);
    }
};

int solve(){
  int n,m;cin>>n>>m;

  vector<iii> E;
  rep(i, m){
    int L,R,C;cin>>L>>R>>C;
    L--;R--;
    E.pb(mt(L, R, C));
  }
  sort(all(E));
  dump(E);

  vi L(m);
  vi R(m);
  vi C(m);
  rep(i, m){
    L[i] = get<0>(E[i]);
    R[i] = get<1>(E[i]);
    C[i] = get<2>(E[i]);
  }
  dump(L);
  dump(R);
  dump(C);

  vi dp(n+1, INF);
  dp[0] = 0;

  SegmentTree sg(n+1);
  sg.update(0, 0);
  rep(i, m){
    int lv = sg.getmin(L[i], n+1);
    int v = lv+C[i];
    int rv = sg.getmin(R[i], n+1);
    dump(lv, v, rv);

    if (v<rv){
      dump(R[i], v);
      sg.update(R[i], v);
    }
  }
  int ans = sg.getmin(n-1, n+1);
  if (ans==INF){
    cout << -1 << endl;
  }
  else{
    cout << ans << endl;
  }

  return 0;
}

signed main() { //{{{
#ifdef INPUT_FROM_FILE
  std::ifstream in(infile);
  std::cin.rdbuf(in.rdbuf());
#endif
  solve();
  return 0;
} //}}}


Measured memory_bytes: 8432