#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cctype>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>
#include<complex>

using namespace std;

#define ll long long
#define ld long double
#define EPS 1e-9
#define INF 1e9
#define LINF (ll)INF*INF
#define MOD 1000000007
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,a,n) for(int i=a;i<(n);i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

#define int ll //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int,int> pii;
typedef pair<pii,int> ppi;
typedef pair<int,pii> pip;
typedef vector<pii> vp;
typedef vector<vi> vvi;

int gcd(int a, int b){if(b==0) return a;return gcd(b,a%b);}
int lcm(int a, int b){return a/gcd(a,b)*b;}

class LazySegmentTree{
 private:
     int n;
     vector<long long> node, lazy;
     //long long node_init = 0; // sum, max, update
     long long node_init = LINF; // min
     //long long lazy_init = 0; // default
     long long lazy_init = LINF; // min, update (存在しない値)

     long long lazy_update(long long a,long long b){
         //return a+b; // sum query
         //return max(a,b); // max query
         return min(a,b); // min query
         //return b; // update query
     }

     long long node_update(long long a, long long b, int l, int r){ // lazy を node に反映
         return min(a,b);
        //return a + b; // min query
         //return b; // update query
         //return b * (r-l); // sum query
     }

     long long combine(long long a,long long b){
         //return a+b; // sum query
         //return max(a,b); // max query
         return min(a,b); // min query
     }

 public:
     LazySegmentTree(){}
     LazySegmentTree(vector<long long> in){
         n = 1;
         while(n < in.size())n <<= 1;
         node = vector<long long>(2*n, node_init);
         lazy = vector<long long>(2*n, lazy_init);
         for(int i = n-1+in.size()-1; i >= 0; i--){
             if(n-1 <= i)node[i] = in[i-(n-1)];
             else node[i] = combine(node[i*2+1], node[i*2+2]);
         }
     }
     void eval(int k, int l, int r){
         if(lazy[k] != lazy_init){
             node[k] = node_update(node[k], lazy[k], l, r);
             if(r - l > 1){
                 lazy[2*k+1] = lazy_update(lazy[2*k+1], lazy[k]);
                 lazy[2*k+2] = lazy_update(lazy[2*k+2], lazy[k]);
             }
             lazy[k] = lazy_init;
         }
     }
     void update(int a, int b, long long x, int k = 0, int l = 0, int r = -1){
         if(r < 0)r = n;
         eval(k,l,r);
         if(b <= l || r <= a)return;
         if(a <= l && r <= b){
             lazy[k] = x;
             eval(k,l,r);
         }else{
             update(a,b,x,2*k+1,l,(l+r)/2);
             update(a,b,x,2*k+2,(l+r)/2,r);
             node[k] = combine(node[2*k+1], node[2*k+2]);
         }
     }
     long long query(int a, int b, int k = 0, int l = 0, int r = -1){
         if(r < 0)r = n;
         if(b <= l || r <= a)return node_init;
         eval(k,l,r);
         if(a <= l && r <= b)return node[k];
         return combine(query(a,b,2*k+1,l,(l+r)/2), query(a,b,2*k+2,(l+r)/2,r));
     }
     void show(){
         cout << "node :";
         for(int i = 0; i < 2*n-1; i++){
             if(i == n-1)cout << "     ";
             cout << " " << node[i];
         }
         cout << endl;
         cout << "lazy :";
         for(int i = 0; i < 2*n-1; i++){
             if(i == n-1)cout << "     ";
             cout << " " << lazy[i];
         }
         cout << endl;
     }
 };

signed main(void) {
    int n,m;
    cin >> n >> m;
    vector<ppi> in(m);
    rep(i,m){
        cin >> in[i].first.first >> in[i].first.second >> in[i].second;
        in[i].first.first--;
        in[i].first.second--;

    }
    sort(all(in));
    vi d(n,LINF);
    d[0] = 0;
    LazySegmentTree st(d);
    priority_queue< pii, vector<pii>, greater<pii> > q;
    int ind = 0;
    rep(i,n){
        int now = st.query(i,i+1);
        //cout << i << " " << now << endl;
        while(ind < m && in[ind].first.first <= i){
            int ne = in[ind].first.second;
            int c = in[ind].second;
            //cout << "?" << ne << " " << c << endl;
            ind++;
            // [i+1,ne]に対して、min(行き先, d[i]+c);
            st.update(i+1,ne+1,now+c);
        }
    }
    int ans = st.query(n-1,n);
    if(ans < LINF){
        cout << ans << endl;
    }else{
        cout << -1 << endl;
    }
}


Measured memory_bytes: 8192