#include <bits/stdc++.h>
#include <iomanip>
#include <math.h>
#include <unistd.h>
#include <stdio.h>
#define all(a) (a).begin(),(a).end()
#define vecin(a) rep(i,a.size())cin >> a[i]
#define vecout(a) rep(i,a.size()){cout << a[i];cout << (i == a.size() - 1 ? "\n":" ");}
#define rep(i,x) for(ll i = 0;i<x;i++)
#define REP(i,x) for(ll i = 1;i<=x;i++)
#define mp make_pair
using ll = long long;
using ld = long double;
using namespace std;
using dou = double;
const ll inf = 2147483647;
const ll INF = 1LL << 60;
const dou pi = 3.14159265358;
const ll mod = 1000000007LL;
//const ll mod = 998244353LL;
typedef pair<ll,ll> P;
using graph = vector<vector<ll>>;
template<class T> inline bool chmax(T& a, T b){if(a < b){a = b; return true;}return false;}
template<class T> inline bool chmin(T& a, T b){if(a > b){a = b; return true;}return false;}
template<class T> inline bool change(T a,T b){if(a > b){swap(a,b);return true;}return false;}
template<class T>
T gcd(T a,T b){
    if(a < b)swap(a,b);
    if(a % b == 0)return b;
    else return gcd(b,a%b);
}
template<class T>
T lcm(T a,T b){
    return a / gcd(a,b) * b;
}

//素数判定O(sqrt(N))
template<class T>
inline bool isp(T n){
    bool res = true;
    if(n == 1)return false;
    else{
        for(ll i = 2;i * i <= n;i++){
            if(n % i == 0){
                res = false;
                break;
            }
        }
        return res;
    }
}
const ll cmax = 1000000;
ll fac[cmax], finv[cmax], inv[cmax];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i < cmax; i++){
        fac[i] = fac[i - 1] * i % mod;
        inv[i] = mod - inv[mod%i] * (mod / i) % mod;
        finv[i] = finv[i - 1] * inv[i] % mod;
    }
}

// 二項係数計算
ll nCk(ll n, ll k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}
//nのm乗をMODで割ったあまりO(logm)
ll modpow(ll n,ll m,ll MOD){
    if(m == 0)return 1;
    if(m < 0)return -1;
    ll res = 1;
    while(m){
        if(m & 1)res = (res * n) % MOD;
        m >>= 1;
        n *= n;
        n %= MOD;
    }
    return res;
}
ll mypow(ll n,ll m){
    if(m == 0)return 1;
    if(m < 0)return -1;
    ll res = 1;
    while(m){
        if(m & 1)res = (res * n);
        m >>= 1;
        n *= n;
    }
    return res;
}

#define fir first
#define sec second
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);
    ll N,M;
    cin >> N >> M;
    struct edge{ll to,cost;};
    vector<edge> es[100005];
    ll a,b,c;
    rep(i,M){
        cin >> a >> b >> c;
        a--;b--;
        es[a].push_back({b,c});
        es[b].push_back({a,c});
    }
    rep(i,N - 1){
        es[i + 1].push_back({i,0});
    }
    priority_queue<P,vector<P>,greater<P>> que;
    que.push(P(0,0));
    vector<ll> dist(N,INF);
    dist[0] = 0;
    while(que.size()){
        P p = que.top();
        que.pop();
        if(dist[p.sec] < p.fir)continue;
        for(auto x : es[p.sec]){
            if(chmin(dist[x.to],dist[p.sec] + x.cost)){
                que.push(P(dist[x.to],x.to));
            }
        }
    }
    if(dist[N - 1] == INF)cout << "-1\n";
    else cout << dist[N - 1] << "\n";
}

Measured memory_bytes: 15636