#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cerr << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cerr<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 2000010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = (1 << 30) - 1;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
mt19937 rng; //use it by rng() % mod, shuffle(all(vec), rng)
///////////////////////////////////////////////////////////////////////////////////////////////////

int N, M, n;
vector<pi> G[MAX_N];

void add_edge(int a, int b, ll c, int idx, int k, int l, int r) {
	// debug(a, b, c, k, l, r);
	if(b <= l || r <= a) return;
	if(a <= l && r <= b) {
		G[k + 2 * n].pb(pi(idx, c));
		G[idx].pb(pi(k, 0));
	}
	else {
		add_edge(a, b, c, idx, k * 2 + 1, l, (l + r) / 2);
		add_edge(a, b, c, idx, k * 2 + 2, (l + r) / 2, r);
	}
}

ll d[MAX_N];

void dijkstra(int s) {
	//can be used as bfs.
	priority_queue<pl, vector<pl>, greater<pl> > que;
	fill(d, d + 4 * n + M, linf);
	d[s] = 0;
	que.push(pl(0, s));
	while(!que.empty()) {
		pl p = que.top(); que.pop();
		int v = p.second;
		if(d[v] < p.first) continue;
		for(int i = 0; i < sz(G[v]); i++) {
			pi e = G[v][i];
			if(d[e.fst] > d[v] + e.sec) {
				d[e.fst] = d[v] + e.sec;
				que.push(pl(d[e.fst], e.fst));
			}
		}
	}
}

void solve() {
	cin >> N >> M;
	n = 1;
	while(n < N) n *= 2;

	rep(i, 0, n - 1) {
		G[i].pb(pi(i * 2 + 1, 0));
		G[i].pb(pi(i * 2 + 2, 0));
		G[i * 2 + 1 + 2 * n].pb(pi(i + 2 * n, 0));
		G[i * 2 + 2 + 2 * n].pb(pi(i + 2 * n, 0));
	}
	for(int i = n - 1; i < 2 * n - 1; i++) {
		G[i].pb(pi(i + 2 * n, 0));
	}
	rep(i, 0, M) {
		int l, r, c; cin >> l >> r >> c; l--;
		add_edge(l, r, c, 4 * n + i, 0, 0, n);
	}
	dijkstra(n - 1);
	// rep(i, 0, 4 * n) {
	// 	debug(i, vector<pl>(all(G[i])), d[i]);
	// }
	if(d[n - 1 + N - 1] == linf) cout << -1 << "\n";
	else cout << d[n - 1 + N - 1] << "\n";
}

uint32_t rd() {
	uint32_t res;
#ifdef __MINGW32__
	asm volatile("rdrand %0" :"=a"(res) ::"cc");
#else
	res = std::random_device()();
#endif
	return res;
}

int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
    cerr << fixed;
	cerr.precision(6);
	rng.seed(rd());
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
	if(!freopen("in.txt", "rt", stdin)) return 1;
#endif	
	solve();
    cerr << "Time: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
	return 0;
}



Measured memory_bytes: 105076