/*** author: yuji9511 ***/
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using lpair = pair<ll, ll>;
const ll MOD = 1e9+7;
const ll INF = 1e18;
#define rep(i,m,n) for(ll i=(m);i<(n);i++)
#define rrep(i,m,n) for(ll i=(m);i>=(n);i--)
#define printa(x,n) for(ll i=0;i<n;i++){cout<<(x[i])<<" \n"[i==n-1];};
void print() {}
template <class H,class... T>
void print(H&& h, T&&... t){cout<<h<<" \n"[sizeof...(t)==0];print(forward<T>(t)...);}
#define N 100003
 
class SegmentTree {
  static const ll inf = 1e18;
  struct Node {
    Node *left, *right;
    ll max_v, smax_v, max_c;
    ll min_v, smin_v, min_c;
    ll sum;
    ll len, ladd, lval;

    Node() : left(0), right(0), ladd(0), lval(inf) {}

    void init(ll x) {
      max_v = min_v = sum = x;
      smax_v = -inf; smin_v = inf;
      max_c = min_c = 1;
    }

    void init_empty() {
      max_v = smax_v = -inf;
      min_v = smin_v = inf;
      max_c = min_c = 0;
    }

    void update_max(ll x) {
      sum += (x - max_v) * max_c;

      if(max_v == min_v) {
        max_v = min_v = x;
      } else if(max_v == smin_v) {
        max_v = smin_v = x;
      } else {
        max_v = x;
      }

      if(lval != inf && x < lval) {
        lval = x;
      }
    }

    void update_min(ll x) {
      sum += (x - min_v) * min_c;

      if(max_v == min_v) {
        max_v = min_v = x;
      } else if(max_v == smin_v) {
        min_v = smax_v = x;
      } else {
        min_v = x;
      }

      if(lval != inf && lval < x) {
        lval = x;
      }
    }

    void addall(ll x) {
      max_v += x;
      if(smax_v != -inf) smax_v += x;
      min_v += x;
      if(smin_v != inf) smin_v += x;

      sum += len * x;
      if(lval != inf) {
        lval += x;
      } else {
        ladd += x;
      }
    }

    void updateall(ll x) {
      max_v = min_v = x;
      smax_v = -inf; smin_v = inf;
      max_c = min_c = len;

      sum = len * x;
      lval = x; ladd = 0;
    }

    void push() {

      if(lval != inf) {
        left->updateall(lval);
        right->updateall(lval);
        lval = inf;
        return;
      }

      if(ladd != 0) {
        left->addall(ladd);
        right->addall(ladd);
        ladd = 0;
      }

      if(max_v < left->max_v) {
        left->update_max(max_v);
      }
      if(left->min_v < min_v) {
        left->update_min(min_v);
      }

      if(max_v < right->max_v) {
        right->update_max(max_v);
      }
      if(right->min_v < min_v) {
        right->update_min(min_v);
      }
    }

    void update() {
      sum = left->sum + right->sum;

      if(left->max_v < right->max_v) {
        max_v = right->max_v;
        max_c = right->max_c;
        smax_v = max(left->max_v, right->smax_v);
      } else if(left->max_v > right->max_v) {
        max_v = left->max_v;
        max_c = left->max_c;
        smax_v = max(left->smax_v, right->max_v);
      } else {
        max_v = left->max_v;
        max_c = left->max_c + right->max_c;
        smax_v = max(left->smax_v, right->smax_v);
      }

      if(left->min_v < right->min_v) {
        min_v = left->min_v;
        min_c = left->min_c;
        smin_v = min(left->smin_v, right->min_v);
      } else if(left->min_v > right->min_v) {
        min_v = right->min_v;
        min_c = right->min_c;
        smin_v = min(left->min_v, right->smin_v);
      } else {
        min_v = left->min_v;
        min_c = left->min_c + right->min_c;
        smin_v = min(left->smin_v, right->smin_v);
      }
    }
  };

  int n, n0;
  Node *root;

  void _update_min(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a || nd->max_v <= x) {
      return;
    }
    if(a <= l && r <= b && nd->smax_v < x) {
      nd->update_max(x);
      return;
    }

    nd->push();
    _update_min(x, a, b, nd->left, l, (l+r)/2);
    _update_min(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  void _update_max(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a || x <= nd->min_v) {
      return;
    }
    if(a <= l && r <= b && x < nd->smin_v) {
      nd->update_min(x);
      return;
    }

    nd->push();
    _update_max(x, a, b, nd->left, l, (l+r)/2);
    _update_max(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  void _add_val(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return;
    }
    if(a <= l && r <= b) {
      nd->addall(x);
      return;
    }

    nd->push();
    _add_val(x, a, b, nd->left, l, (l+r)/2);
    _add_val(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  void _update_val(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return;
    }
    if(a <= l && r <= b) {
      nd->updateall(x);
      return;
    }

    nd->push();
    _update_val(x, a, b, nd->left, l, (l+r)/2);
    _update_val(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  ll _query_max(int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return -inf;
    }
    if(a <= l && r <= b) {
      return nd->max_v;
    }
    nd->push();
    ll lv = _query_max(a, b, nd->left, l, (l+r)/2);
    ll rv = _query_max(a, b, nd->right, (l+r)/2, r);
    return max(lv, rv);
  }

  ll _query_min(int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return inf;
    }
    if(a <= l && r <= b) {
      return nd->min_v;
    }
    nd->push();
    ll lv = _query_min(a, b, nd->left, l, (l+r)/2);
    ll rv = _query_min(a, b, nd->right, (l+r)/2, r);
    return min(lv, rv);
  }

  ll _query_sum(int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return 0;
    }
    if(a <= l && r <= b) {
      return nd->sum;
    }
    nd->push();
    ll lv = _query_sum(a, b, nd->left, l, (l+r)/2);
    ll rv = _query_sum(a, b, nd->right, (l+r)/2, r);
    return lv + rv;
  }

public:
  SegmentTree(int n, ll *a) : n(n) {
    n0 = 1;
    while(n0 < n) n0 <<= 1;

    Node *nds = new Node[2*n0];
    root = nds;

    nds[0].len = n0;
    for(int i=0; i<n0-1; ++i) {
      nds[i].left = &nds[2*i+1];
      nds[i].right = &nds[2*i+2];
      nds[2*i+1].len = nds[2*i+2].len = (nds[i].len >> 1);
    }

    for(int i=0; i<n; ++i) nds[n0-1+i].init(a[i]);
    for(int i=n; i<n0; ++i) nds[n0-1+i].init_empty();
    for(int i=n0-2; i>=0; i--) nds[i].update();
  }

  void update_min(int a, int b, ll x) {
    _update_min(x, a, b, root, 0, n0);
  }

  void update_max(int a, int b, ll x) {
    _update_max(x, a, b, root, 0, n0);
  }

  void add_val(int a, int b, ll x) {
    _add_val(x, a, b, root, 0, n0);
  }

  void update_val(int a, int b, ll x) {
    _update_val(x, a, b, root, 0, n0);
  }

  ll query_max(int a, int b) {
    return _query_max(a, b, root, 0, n0);
  }

  ll query_min(int a, int b) {
    return _query_min(a, b, root, 0, n0);
  }

  ll query_sum(int a, int b) {
    return _query_sum(a, b, root, 0, n0);
  }
};

ll L[100010], R[100010], C[100010];

typedef struct {
    ll L;
    ll R;
    ll C;
} P;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll N1,M;
    cin >> N1 >> M;
    rep(i,0,M){
        cin >> L[i] >> R[i] >> C[i];
        L[i]--; R[i]--;
    }
    ll v[100010] = {};
    rep(i,1,N) v[i] = INF;
    SegmentTree stb(N, v);
    vector<P> lp;
    rep(i,0,M) lp.push_back((P){L[i], R[i], C[i]});
    sort(lp.begin(), lp.end(), [](P p1, P p2){
        if(p1.L == p2.L){
            return p1.R < p2.R;
        }
        return p1.L < p2.L;
    });

    ll dist[100010] = {};
    dist[0] = 0;
    rep(i,1,N1) dist[i] = INF;
    rep(i,0,M){
        // ll d1 = dist[lp[i].L];
        ll d1 = stb.query_sum(lp[i].L, lp[i].L+1);
        stb.update_min(lp[i].L+1, lp[i].R+1, d1 + lp[i].C);

        // rep(j,lp[i].L+1,lp[i].R+1){
        //     dist[j] = min(dist[j], d1 + lp[i].C);
        // }
    }
    ll ans = stb.query_sum(N1-1, N1);
    if(ans == INF){
        print(-1);
    }else{
        print(ans);
    }
    

}

Measured memory_bytes: 32500