#include <bits/stdc++.h>
using namespace std;
struct segment {
  int l, u;
} tmp;
pair<int, int> seg[4 * int(6e5 + 10)], lazy[4 * int(6e5 + 10)], x;
int p[int(6e5 + 10)], n, m, k, dp;
pair<int, int> ans = {-1, -1};
vector<segment> g[int(6e5 + 10)];
set<int> s;
map<int, int> mp;
bool vis[int(6e5 + 10)];
pair<int, int> fmax(pair<int, int> a, pair<int, int> b) {
  if (a.first < b.first) return b;
  return a;
}
void build(int l, int h, int pos) {
  if (l == h) {
    seg[pos] = lazy[pos] = {0, 0};
    return;
  }
  int mid = (l + h) / 2;
  build(l, mid, 2 * pos);
  build(mid + 1, h, 2 * pos + 1);
  seg[pos] = lazy[pos] = {0, 0};
}
void update(int l, int h, int x, int y, pair<int, int> z, int pos) {
  if (lazy[pos].first) {
    seg[pos] = fmax(seg[pos], lazy[pos]);
    if (l != h) {
      lazy[2 * pos] = fmax(lazy[pos], lazy[2 * pos]);
      lazy[2 * pos + 1] = fmax(lazy[pos], lazy[2 * pos + 1]);
    }
    lazy[pos] = {0, 0};
  }
  if (h < x || l > y) return;
  if (x <= l && h <= y) {
    seg[pos] = fmax(seg[pos], z);
    if (l != h) {
      lazy[2 * pos] = fmax(lazy[2 * pos], z);
      lazy[2 * pos + 1] = fmax(lazy[2 * pos], z);
    }
    return;
  }
  int mid = (l + h) / 2;
  update(l, mid, x, y, z, 2 * pos);
  update(mid + 1, h, x, y, z, 2 * pos + 1);
  seg[pos] = fmax(seg[2 * pos], seg[2 * pos + 1]);
}
pair<int, int> query(int l, int h, int x, int y, int pos) {
  if (lazy[pos].first) {
    seg[pos] = fmax(seg[pos], lazy[pos]);
    if (l != h) {
      lazy[2 * pos] = fmax(lazy[pos], lazy[2 * pos]);
      lazy[2 * pos + 1] = fmax(lazy[pos], lazy[2 * pos + 1]);
    }
    lazy[pos] = {0, 0};
  }
  if (h < x || l > y) return {-1, -1};
  if (x <= l && h <= y) {
    return seg[pos];
  }
  int mid = (l + h) / 2;
  return fmax(query(l, mid, x, y, 2 * pos),
              query(mid + 1, h, x, y, 2 * pos + 1));
}
void fprint(int t) {
  if (p[t] != t) {
    fprint(p[t]);
  }
  vis[t] = true;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cout.precision(10);
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    cin >> k >> tmp.l >> tmp.u;
    s.insert(tmp.l);
    s.insert(tmp.u);
    g[k].push_back(tmp);
  }
  k = 1;
  for (auto x : s) {
    if (mp.find(x) == mp.end()) {
      mp[x] = k++;
    }
  }
  build(1, k, 1);
  for (int i = 1; i <= n; i++) {
    p[i] = i;
    dp = 1;
    for (auto s : g[i]) {
      x = query(1, k, mp[s.l], mp[s.u], 1);
      if (dp < 1 + x.first) {
        p[i] = x.second;
        dp = 1 + x.first;
      }
    }
    for (auto s : g[i]) {
      update(1, k, mp[s.l], mp[s.u], make_pair(dp, i), 1);
    }
    ans = fmax(ans, make_pair(dp, i));
  }
  cout << n - ans.first << endl;
  fprint(ans.second);
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) cout << i << ' ';
  }
  cout << endl;
  return 0;
}
