#include <bits/stdc++.h>
using namespace std;
using ll = long long;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n, t;
  cin >> n >> t;
  vector<pair<int, int>> q(t);
  for (int i = 0; i < t; ++i) {
    cin >> q[i].first >> q[i].second;
  }
  int ans = 0;
  for (int i = 0; i < t; ++i) {
    int pref[n + 2], npref[n + 2];
    memset(pref, 0, sizeof(pref));
    for (int k = 0; k < t; ++k) {
      if (i == k) continue;
      pref[q[k].first]++;
      pref[q[k].second + 1]--;
    }
    int cnt = 0;
    for (int i = 1; i <= n + 1; ++i) {
      pref[i] += pref[i - 1];
      if (pref[i]) ++cnt;
    }
    npref[0] = 0;
    for (int i = 1; i <= n + 1; ++i) {
      npref[i] = npref[i - 1];
      if (pref[i] == 1) {
        npref[i]++;
      }
    }
    for (int k = 0; k < t; ++k) {
      if (k == i) continue;
      int l = q[k].first, r = q[k].second;
      ans = max(ans, cnt - npref[r] + npref[l - 1]);
    }
  }
  cout << ans << '\n';
  ;
  return 0;
}
