#include <bits/stdc++.h>
using namespace std ;
//#define int long long
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define loop(it, v) for (auto it = v.begin(); it != v.end(); it++)
#define cont(i, x) for (int i = head[x], to = e[i].to; i; i = e[i].nxt, to = e[i].to)
#define clr(a) memset(a, 0, sizeof(a))
#define ass(a, sum) memset(a, sum, sizeof(a))
#define lowbit(x) (x & -x)
#define all(x) x.begin(), x.end()
#define ub upper_bound
#define lb lower_bound
#define pqueue priority_queue
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define enter cout << endl
#define siz(x) ((int)x.size())
#define file(x) freopen(x".in", "r", stdin),freopen(x".out", "w", stdout)
typedef long double ld ;
typedef long long ll ;
typedef unsigned long long ull ;
typedef pair <int, int> pii ;
typedef pair <ll, int> pli ;
typedef pair<int, pii> tri ;
typedef vector <int> vi ;
typedef vector <pii> vii ;
typedef vector <vi> vvi ;
typedef vector <string> vst ;
typedef queue <int> qi ;
typedef queue <pii> qii ;
typedef map <int, int> mii ;
typedef map <string, int> msi ;
const int N = 10010 ;
const int M=  200010 ;
const int INF = 0x3f3f3f3f ;
const int iinf = 1 << 30 ;
const ll linf = 2e18 ;
const int mod = 1000000007 ;
const double eps = 1e-7 ;
void douout(double x){ printf("%lf\n", x + 0.0000000001) ; }
template <class T> void print(T a) { cout << a << endl ; exit(0) ; }
template <class T> void chmin(T &a, T b) { if (a > b) a = b ; }
template <class T> void chmax(T &a, T b) { if (a < b) a = b ; }
template <class T> void Tmin(T &a, T &b, T &c) { return min(a, min(b, c)) ; }
template <class T> void Tmax(T &a, T &b, T &c) { return max(a, max(b, c)) ; }
void add(int &a, int b) { a = a + b < mod ? a + b : a + b - mod ; }
void sub(int &a, int b) { a = (a - b + mod) % mod ; }
void mul(int &a, int b) { a = (ll) a * b % mod ; }
int addv(int a, int b) { return (a += b) >= mod ? a -= mod : a ; }
int subv(int a, int b) { return (a -= b) < 0 ? a += mod : a ; }
int mulv(int a, int b) { return (ll) a * b % mod ; }
int read() {
    int f = 1, x = 0 ;
    char ch = getchar() ;
    while (!isdigit(ch)) { if (ch == '-') f = -1 ; ch = getchar() ; }
    while (isdigit(ch)) { x = x * 10 + ch -'0' ; ch = getchar() ; }
    return x * f ;
}
int pw(int a, int b) {
    int s = 1 ;
    for (; b; b >>= 1, a = (ll) a * a % mod)
    if (b & 1) s = (ll) s * a % mod ;
    return s ;
}

// 考前检查：文件，注释，数组，ll, 编译

int n, m ;

struct Edge {
	int to, nxt ;
	ll dis ;
} e[M * 2] ;

int head[M], tot ;
ll dis[M] ;
int vis[M] ;

void add(int a, int b, ll dis) {
	e[++tot] = (Edge) {b, head[a], dis} ;
	head[a] = tot ;
}

struct node {
	int x ; ll val ;
	node() {}
	node(int x, ll val) : x(x), val(val) {}
	bool operator < (const node & rhs) const {
		return val > rhs.val ;
	} 
};	

void dij(int x) {
	clr(vis) ; dis[x] = 0 ;
	pqueue <node> q ; q.push(node(x, 0)) ;
	while (!q.empty()) {
		int x = q.top().x ; q.pop() ;
		if (vis[x]) continue ;
		vis[x] = 1 ;
		cont(i, x) {
			if (dis[to] > dis[x] + e[i].dis) {
				dis[to] = dis[x] + e[i].dis ;
				q.push(node(to, dis[to])) ;
			}
		}
	}
}

void init(int n){
    clr(head) ;
    tot = 0 ;
    rep(i, 1, n) dis[i] = linf ;
}

signed main() {
	scanf("%d%d", &n, &m) ;
	init(n) ;
	rep(i, 1, n) add(i, i - 1, 0) ;
	rep(i, 1, m) {
		int u, v ; ll dis ; 
		scanf("%d%d%lld", &u, &v, &dis) ;
		add(u, v, dis) ;
	}
	dij(1) ;
	if (dis[n] == linf) puts("-1") ;
	else printf("%lld\n", dis[n]) ; 
}

Measured memory_bytes: 7936