#include <bits/stdc++.h>
#include <random>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef double ld;
#define int ll
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<vvc> vvvc;
typedef pair<int, int> pii;
typedef pair<pii, pii> piii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii> vpi;
typedef vector< vi > vvi;
typedef vector< vvi > vvvi;
typedef vector<short> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<ld> vld;
typedef vector<vld> vvld;
typedef vector<vvld> vvvld;
typedef vector<string> vst;
typedef vector<vst> vvst;
typedef pair<ld, ld> pld;

#define inmin(a, b) a = min(a, (b))
#define inmax(a, b) a = max(a, (b))
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define sqr(x) ((x) * (x))
#define fori(i, n) for(int i = 0; i < int(n); ++i)
#define SZ(a) ((int)((a).size()))
#define triple(T) tuple<T, T, T>
#define quad(T) tuple<T, T, T, T>
#define watch(x) cerr << (#x) << " = " << (x) << endl;

#ifdef RUS_HOME
#define cerr cout
#else
#define cerr if (false) cerr
#endif

const double PI = 2 * acos(0.0);
#define rand shittttty_shit
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";


template <class T0, class T1>
inline ostream & operator << (ostream &out, pair<T0, T1> &a) {
    return out << "{" << a.first << ", " << a.second << "}";
}

template <class T0, class T1>
inline istream & operator >> (istream &in, pair<T0, T1> &a) {
    return in >> a.first >> a.second;
}

template <class T0, class T1, class T2>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
}

template <class T0, class T1, class T2, class T3>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2, T3> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " <<  get<3>(a) << "}";
}

template<class T>
inline ostream & operator << (ostream &out, vector<T> &a) {
    out << "[";
    fori (i, a.size())
        out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
    return out;
}


void smain();



signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
#ifdef RUS_HOME
    freopen("input", "r", stdin);
    clock_t start = clock();
#endif
    cout << setprecision(12) << fixed;
    smain();
#ifdef RUS_HOME
    cout << "\n\n\n\nTOTAL EXECUTION TIME: " << float( clock () - start ) /  CLOCKS_PER_SEC << endl;
#endif
    return 0;
}

const int MOD=998244353,N=1e5+100,oo=1e15;

vi t(4*N,oo);

void upd(int v,int tl,int tr,int pos,int val){
    if(tl==tr){
        inmin(t[v],val);
        return;
    }
    int tm=tl+tr>>1;
    if(pos<=tm)
        upd(v<<1,tl,tm,pos,val);
    else
        upd(v<<1|1,tm+1,tr,pos,val);
    t[v]=min(t[v<<1],t[v<<1|1]);
}

int get(int v,int tl,int tr,int l,int r){
    if(l>r)
        return oo;
    if(tl==l&&r==tr)
        return t[v];
    int tm=tl+tr>>1;
    return min(get(v<<1,tl,tm,l,min(r,tm)),
            get(v<<1|1,tm+1,tr,max(tm+1,l),r));
}


void smain() {

    int n,m;
    cin>>n>>m;
    vector<pair<pii,int>> kek(m);
    for(int i=0;i<m;i++){
        int l,r,c;
        cin>>l>>r>>c;
        kek[i]={{l,r},c};
    }

    sort(ALL(kek));
    upd(1,1,n,1,0);
    for(int i=0;i<m;i++){
        int l=kek[i].first.first,r=kek[i].first.second,c=kek[i].second;
        int val=get(1,1,n,l,r-1);
        upd(1,1,n,r,val+c);
    }

    int ans=get(1,1,n,n,n);
    if(ans==oo)
        cout<<-1;
    else
        cout<<ans;







}

Measured memory_bytes: 8716