#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define REPS(i,x) for(int i=1;i<=(int)(x);i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RREPS(i,x) for(int i=((int)(x));i>0;i--)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define ALL(container) (container).begin(), (container).end()
#define RALL(container) (container).rbegin(), (container).rend()
#define mp(a,b) make_pair(a, b)
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define __builtin_popcount __builtin_popcountll
#define endl '\n'

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 (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
    os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const map<S, T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class S, class T> pair<S,T> operator+(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first+t.first, s.second+t.second);}
template<class S, class T> pair<S,T> operator-(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first-t.first, s.second-t.second);}

const ll INF = 1ll<<60;
const double EPS = 1e-8;
const int MOD = 998244353;

struct handler{
    typedef ll val_t;
    typedef ll opr_t;
    handler(){}
    val_t def_val(){ return INF; }
    opr_t def_lazy(){ return INF; }
    static val_t resolve(const val_t &x, const opr_t &y, int l, int r){
        return min(x, y);
    }
    static opr_t push(const opr_t &x, const opr_t &y){
        return min(x, y);
    }
    static val_t merge(const val_t &x, const val_t &y){
        return min(x, y);
    }
};

template<typename Handler>
struct SegTreeLazy{
    typedef typename Handler::val_t val_t;
    typedef typename Handler::opr_t opr_t;
    vector<val_t> val;
    vector<opr_t> lazy;
    Handler hdl;
    int n;

    SegTreeLazy(int size):hdl(){
        n=1;
        while(n<size) n<<=1;
        val=vector<val_t>(2*n, hdl.def_val());
        lazy=vector<opr_t>(2*n, hdl.def_lazy());
    }

    SegTreeLazy(const vector<val_t> &in):hdl(){
        n=1;
        while(n<in.size()) n<<=1;
        val=vector<val_t>(2*n, hdl.def_val());
        lazy=vector<opr_t>(2*n, hdl.def_lazy());
        for(int i=n-1 + in.size()-1;i>=0;i--){
            if(n-1 <= i) val[i] = in[i - (n-1)];
            else val[i] = hdl.merge(val[i*2+1],val[i*2+2]);
        }
    }
    val_t update(int a,int b,opr_t x, int k,int l,int r){
        if(r<=a||b<=l) return hdl.resolve(val[k], lazy[k], l, r);
        if(a<=l&&r<=b){
            lazy[k] = hdl.push(lazy[k], x);
            return hdl.resolve(val[k], lazy[k], l, r);
        }
        lazy[k*2+1] = hdl.push(lazy[k*2+1], lazy[k]);
        lazy[k*2+2] = hdl.push(lazy[k*2+2], lazy[k]);
        lazy[k] = hdl.def_lazy();
        return val[k] = hdl.merge(update(a, b, x, k*2+1, l, (l+r)/2),
                                  update(a, b, x, k*2+2, (l+r)/2, r));
    }
    val_t query(int a,int b,int k,int l,int r){
        if(r<=a||b<=l) return hdl.def_val();
        if(a<=l&&r<=b) return hdl.resolve(val[k], lazy[k], l, r);
        val[k] = hdl.resolve(val[k], lazy[k], l, r);
        lazy[k*2+1] = hdl.push(lazy[k*2+1], lazy[k]);
        lazy[k*2+2] = hdl.push(lazy[k*2+2], lazy[k]);
        lazy[k] = hdl.def_lazy();
        return hdl.merge(query(a, b, k*2+1, l, (l+r)/2),
                         query(a, b, k*2+2, (l+r)/2, r));
    }
    val_t update(int a, int b, opr_t x){return update(a, b, x, 0, 0, n);}
    val_t query(int a, int b){return query(a, b, 0, 0, n);}
    val_t operator[](size_t i){return query(i, i+1);}
    friend ostream& operator<<(ostream &os, SegTreeLazy<Handler> &t){
        REP(i, t.n) os << (i ? ", " : "[") << t.query(i, i+1);
        return os << "]";
    }
};


int T, n, m;

int main(int argc, char *argv[]){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    while(cin >> n >> m){
        vector<tuple<int, int, int>> q;
        REP(i, m) {
            int a, b, c;
            cin >> a >> b >> c;
            q.emplace_back(-- a, b, c);
        }
        sort(ALL(q));
        SegTreeLazy<handler> seg(n);
        seg.update(0, 1, 0);
        for (auto it : q) {
            int l, r, c; tie(l, r, c) = it;
            seg.update(l, r, seg.query(l, r) + c);
        }
        ll ans = seg.query(n - 1, n);
        cout << (ans == INF ? -1 : ans) << endl;
    }
    return 0;
}


Measured memory_bytes: 5620