#include <bits/stdc++.h>
#define int long long
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;
constexpr int MOD = 1000000007;
constexpr int INF = numeric_limits<int>::max() / 2;
typedef pair<int,int> P;
using Graph = vector<vector<int>>;
 
template<typename Monoid, typename OperatorMonoid = Monoid>
struct LazySegmentTree{
    using F = function<Monoid(Monoid, Monoid)>;
    using G = function<Monoid(Monoid, OperatorMonoid)>;
    using H = function<OperatorMonoid(OperatorMonoid, OperatorMonoid)>;
    using C = function<OperatorMonoid(OperatorMonoid, int)>;
 
    int sz;
    vector<Monoid> data;
    vector<OperatorMonoid> lazy;
    const F f;
    const G g;
    const H h;
    const Monoid M1;            // モノイドの単位元
    const OperatorMonoid OM0;   // 作用素モノイドの単位元
    const C c;
 
    LazySegmentTree(int n, const F f, const G g, const H h, const Monoid &M1,
        const OperatorMonoid OM0, const C c = [](int a, int b){return a;})
        : f(f), g(g), h(h), M1(M1), OM0(OM0), c(c)
    {
        sz = 1;
        while(sz < n)   sz <<= 1;
        data.assign(sz << 1, M1);
        lazy.assign(sz << 1, OM0);
    }
 
    void set(int k, const Monoid &x){
        data[k + sz] = x;
    }
 
    void build(){
        for(int k = sz - 1; k > 0; --k){
            data[k] = f(data[k << 1], data[k << 1 | 1]);
        }
    }
 
    void propagate(int k, int len){
        if(lazy[k] != OM0){
            if(k < sz){
                lazy[k << 1] = h(lazy[k << 1], lazy[k]);
                lazy[k << 1 | 1] = h(lazy[k << 1 | 1], lazy[k]);
            }
            data[k] = g(data[k], c(lazy[k], len));
            lazy[k] = OM0;
        }
    }
 
    // a,b : 実際の配列上の index
    // k : セグ木上の index
    // l,r : data[k] が実際の配列でのどの区間の両端か (半開区間 [l,r) )
    Monoid update(int a, int b, const OperatorMonoid &x, int k=1, int l=0, int r=-1){
        if(r == -1)     r = sz;
        propagate(k, r - l);
        if(r <= a || b <= l)    return data[k];
        else if(a <= l && r <= b){
            lazy[k] = h(lazy[k], x);
            propagate(k, r - l);
            return data[k];
        }
        else{
            return data[k] = f(update(a, b, x, k << 1, l, (l + r) >> 1),
                                update(a, b, x, k << 1 | 1, (l + r) >> 1, r));
        }
    }
 
    Monoid query(int a, int b, int k=1, int l=0, int r=-1){
        if(r == -1)     r = sz;
        propagate(k, r - l);
        if(r <= a || b <= l)    return M1;
        else if(a <= l && r <= b)   return data[k];
        else{
            return f(query(a, b, k << 1, l, (l + r) >> 1),
                        query(a, b, k << 1 | 1, (l + r) >> 1, r));
        }
    }
 
    Monoid operator[](const int &k){
        return query(k, k + 1);
    }
 
    void print(){
        for(int i = 0; i < data.size(); ++i){
            cerr << data[i] << " ";
        }
        cerr << "\n";
    }
};
 
 
signed main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
 
    int N, M;
    cin >> N >> M;
    int T[M];
    LazySegmentTree<int> cnt(N, [](int a, int b){return min(a, b);},
                        [](int a, int b){return min(a, b);},
                        [](int a, int b){return min(a, b);},
                        INF, INF); 
	pair<P, int> p[M];
	// cnt[0] = 0;
    rep(i,M){
        cin >> p[i].first.first >> p[i].first.second >> p[i].second;
        --p[i].first.first;
    }
	sort(p, p+M);
	cnt.update(0, 1, 0);
 
	rep(i,M){
		cnt.update(p[i].first.first, p[i].first.second, cnt[p[i].first.first]+p[i].second);
	}
	if(cnt[N-1] < INF) cout << cnt[N-1] <<endl;
	else cout << -1 << endl;
}

Measured memory_bytes: 6656