#include<bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < int(n); ++i)
#define rrep(i,n) for (int i = int(n)-1; i >= 0; i--)
#define rep2(i,s,n) for (int i = int(s); i < int(n); ++i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int,int>;
using LP = pair<ll,ll>;
template<class S,class T> istream& operator>>(istream &is,pair<S,T> &p) { return is >> p.first >> p.second; }
template<class S,class T> ostream& operator<<(ostream &os,const pair<S,T> &p) { return os<<'{'<<p.first<<","<<p.second<<'}'; }
template<class T> istream& operator>>(istream &is,vector<T> &v) { for(T &t:v){is>>t;} return is; }
template<class T> ostream& operator<<(ostream &os,const vector<T> &v) { os<<'[';rep(i,v.size())os<<v[i]<<(i==int(v.size()-1)?']':','); return os; }
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
template<class T> bool chmin(T& a,T b) {if(a > b){a = b; return true;} return false;}
template<class T> bool chmax(T& a,T b) {if(a < b){a = b; return true;} return false;}
const int inf = 1001001001;
const ll linf = 1001001001001001001;

template<typename T,typename MERGE,typename UPDATE>
class segtree {
    int n;
    vector<T> val;
    T identity;
    MERGE merge;
    UPDATE _update;

public:
    segtree(int _n,vector<T> init,T identity,
            MERGE merge,UPDATE update)
            :identity(identity),merge(merge),_update(update) {
        n = 1;
        while(n < _n) n *= 2;
        val = vector<T>(2*n-1,identity);
        rep(i,_n) val[i+n-1] = init[i];
        rrep(i,n-1) val[i] = merge(val[i*2+1],val[i*2+2]);
    }
    void update(int i,T x) {
        i += n-1;
        val[i] = _update(val[i],x);
        while(i > 0) {
            i = (i-1)/2;
            val[i] = merge(val[i*2+1],val[i*2+2]);
        }
    }
    // segment [a,b)
    T query(int a,int b,int k = 0,int l = 0,int r = -1) {
        if(r == -1) r = n;
        if(b <= l || r <= a) return identity;
        if(a <= l && r <= b) return val[k];
        T t1 = query(a,b,2*k+1,l,(l+r)/2);
        T t2 = query(a,b,2*k+2,(l+r)/2,r);
        return merge(t1,t2);
    }
    T operator[](int i) const { return val[i]; }
};

struct road {
    int l,r,c;
    road(int l,int r,int c):l(l),r(r),c(c) {}
};
bool operator<(road& a,road& b){ return a.r < b.r; };

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr); cout.tie(nullptr);
    int n,m;
    cin >> n >> m;
    vector<road> v;
    rep(i,m) {
        int l,r,c;
        cin >> l >> r >> c;
        l--; r--;
        v.eb(l,r,c);
    }
    sort(all(v));
    auto f = [](ll a,ll b){ return min(a,b); };
    auto g = [](ll a,ll b){ return min(a,b); };
    segtree<ll,decltype(f),decltype(g)> st(n,vl(n,linf),linf,f,g);
    st.update(0,0);
    int now = 0;
    rep2(i,1,n) {
        while(now < m and v[now].r == i) {
            st.update(i,st.query(v[now].l,i)+v[now].c);
            now++;
        }
    }
    ll ans = st.query(n-1,n);
    cout << (ans == linf ? -1 : ans) << endl;
}


Measured memory_bytes: 7188