#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef set<int> S;
typedef queue<int> Q;
typedef queue<P> QP;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() { int x; scanf("%d",&x); return x;}
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
const int MOD = 1000000007;
#define dame { puts("-1"); return 0;}
#define yn {puts("YES");}else{puts("NO");}

vi dijkstra(v(vp) &G,int V,int s) {
// G[f][i] = P(t,c) means that from `f` to `t` 's cost is `c` 
    vi d(V,INF);
    priority_queue<P,vp, greater<P>> que;
    d[s] = 0;
    que.push(P(0,s));
    while(!que.empty()) {
      P p = que.top();
      que.pop();
      int tar = p.second;
      if(d[tar] < p.first) continue;
      rep(i,sz(G[tar])) {
        P e = G[tar][i];
        if(d[e.first] > d[tar] + e.second) {
          d[e.first] = d[tar] + e.second;
          que.push(P(d[e.first],e.first));
        }
      }
    }
    return d;
  }
  
int main() {
  int N,M;
  cin >> N >> M;
  vv(P) G(N);
  rep(i,M) {
    int l,r,c;
    cin >> l >> r >> c;
    G[l-1].push_back(P(r-1,c));
  }
  rep(i,N-1) {
    G[i+1].push_back(P(i,0));
  }
  vi ans = dijkstra(G,N,0);
  if(ans[N-1] == INF) {
    cout << -1 << endl;
    return 0;
  }
  cout << ans[N-1] << endl;
  return 0;
}