#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
typedef vector<ll> Vec;
typedef vector<int> Vi;
typedef vector<string> Vs;
typedef vector<vector<ll>> VV;

#define REP(i, a, b) for(ll i=(a); i<(b); i++)
#define rep(i, n) REP(i, 0, n)
const ll INF=1e12+18;
const ll MAX=100005;
const ll MOD=1000000007;
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl;
#define YES(n) cout << ((n) ? "YES" : "NO") << endl;
#define ALL(v) v.begin(), v.end()
#define pb(x) push_back(x)
#define mp(a, b) make_pair(a,b)
#define Each(a,b) for(auto &a :b)
#define REPM(i, mp) for (auto i = mp.begin(); i != mp.end(); ++i)
#define dbg(x_) cerr << #x_ << ":" << x_ << endl;
#define dbgmap(mp) cerr << #mp << ":"<<endl; for (auto i = mp.begin(); i != mp.end(); ++i) { cerr << i->first <<":"<<i->second << endl;}
#define dbgarr(n,m,arr) rep(i,n){rep(j,m){cerr<<arr[i][j]<<" ";}cerr<<endl;}
#define dbgdp(n,arr) rep(i,n){cerr<<arr[i]<<" ";}cerr<<endl;
#define sum(v) accumulate(ALL(v),0)
#define fi first
#define se second

template<typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &p) { return s<<"("<<p.first<<", "<<p.second<<")"; }

// vector
template<typename T>
ostream &operator<<(ostream &s, const vector<T> &v) {
    int len=v.size();
    for(int i=0; i<len; ++i) {
        s<<v[i];
        if(i<len-1) s<<"	";
    }
    return s;
}

// 2 dimentional vector
template<typename T>
ostream &operator<<(ostream &s, const vector<vector<T> > &vv) {
    int len=vv.size();
    for(int i=0; i<len; ++i) {
        s<<vv[i]<<endl;
    }
    return s;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n,m;
    cin>>n>>m;

    vector<vector<P>> graph(n);
    ll a,b,c;
    rep(i,m){
        cin>>a>>b>>c;
        a--;
        b--;
        graph[a].pb(mp(b,c));
        graph[b].pb(mp(a,c));
    }
    rep(i,n-1){
        graph[i+1].pb(mp(i,0LL));
    }
    priority_queue<ll, vector<P>, function<bool(P, P)>> q([](P a, P b)->ll {
        return a.second>b.second;
    });

    //dbg(graph);

    Vec d(n,INF);
    q.push(mp(0LL,0LL));
    d[0]=0LL;
    while(!q.empty()){
        P now = q.top();
        q.pop();
        if(d[now.fi]<now.se){
            continue;
        }
//        dbg(now.fi);
//        dbg(now.se);
        // dbg(d);
        Each(elm,graph[now.fi]){
            if(d[elm.fi]>now.se+elm.se){
//                dbg(elm.fi);
//                dbg(now.se);
//                dbg(elm.se);
                d[elm.fi]=now.se+elm.se;
                q.push(mp(elm.fi,now.se+elm.se));
            }
        }
    }
    ll ans= d[n-1]==INF?-1:d[n-1];
    cout<<ans<<endl;
    return  0;
}
