#include <bits/stdc++.h>
using namespace std;
int n, m, it, l, r, dp[300007], vis[300007], cnt, lst[300007];
vector<pair<int, int> > vec[300007];
vector<pair<int, int> > events;
pair<int, int> t[8 * 300007], par, mod[8 * 300007];
map<int, int> mp;
void push(int v) {
  if (mod[v].first > t[2 * v + 1].first) t[2 * v + 1] = mod[v];
  if (mod[v].first > t[2 * v + 2].first) t[2 * v + 2] = mod[v];
  if (mod[v].first > mod[2 * v + 1].first) mod[2 * v + 1] = mod[v];
  if (mod[v].first > mod[2 * v + 2].first) mod[2 * v + 2] = mod[v];
  return;
}
void update(int v, int tl, int tr, int L, int R, int val) {
  if (tr < L || R < tl) return;
  if (L <= tl && tr <= R) {
    if (dp[val] > t[v].first) {
      t[v] = {dp[val], val};
    }
    if (dp[val] > mod[v].first) {
      mod[v] = {dp[val], val};
    }
    return;
  }
  push(v);
  int tm = (tl + tr) >> 1;
  update(2 * v + 1, tl, tm, L, R, val);
  update(2 * v + 2, tm + 1, tr, L, R, val);
  if (t[2 * v + 1].first > t[2 * v + 2].first)
    t[v] = t[2 * v + 1];
  else
    t[v] = t[2 * v + 2];
  return;
}
pair<int, int> get_mx(int v, int tl, int tr, int L, int R) {
  if (tr < L || R < tl) return {-1, -1};
  if (L <= tl && tr <= R) return t[v];
  push(v);
  int tm = (tl + tr) >> 1;
  pair<int, int> mx1 = get_mx(2 * v + 1, tl, tm, L, R);
  pair<int, int> mx2 = get_mx(2 * v + 2, tm + 1, tr, L, R);
  if (mx1.first == -1) return mx2;
  if (mx2.first == -1) return mx1;
  if (mx1.first > mx2.first)
    return mx1;
  else
    return mx2;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    scanf("%d%d%d", &it, &l, &r);
    vec[it].push_back({l, r});
    events.push_back({l, 1});
    events.push_back({r + 1, 0});
  }
  sort(events.begin(), events.end());
  for (int i = 0; i < 2 * m; i++) {
    l = events[i].first;
    it = events[i].second;
    if (mp[l] == 0) mp[l] = ++cnt;
  }
  for (int i = 1; i <= n; i++) {
    dp[i] = 1;
    for (auto p : vec[i]) {
      l = p.first;
      r = p.second;
      l = mp[l];
      r = mp[r + 1] - 1;
      par = get_mx(0, 1, cnt, l, r);
      if (par.first + 1 > dp[i]) {
        dp[i] = par.first + 1;
        lst[i] = par.second;
      }
    }
    for (auto p : vec[i]) {
      l = p.first;
      r = p.second;
      l = mp[l];
      r = mp[r + 1] - 1;
      update(0, 1, cnt, l, r, i);
    }
  }
  par = {0, 0};
  for (int i = 1; i <= n; i++) {
    if (dp[i] > par.first) {
      par.first = dp[i];
      par.second = i;
    }
  }
  printf("%d\n", n - par.first);
  while (par.second != 0) {
    vis[par.second] = 1;
    par.second = lst[par.second];
  }
  for (int i = 1; i <= n; i++)
    if (vis[i] == 0) printf("%d ", i);
  return 0;
}
