#include <bits/stdc++.h>
using namespace std;
int freq[5005], anabas[5005];
short etneen[5001][5001];
vector<vector<short> > cells(5001);
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n, q, ans = 0, rem = 1e9;
  cin >> n >> q;
  vector<pair<int, int> > v(q);
  for (int i = 0; i < q; i++) cin >> v[i].first >> v[i].second;
  for (int i = 0; i < q; i++) {
    for (int j = v[i].first; j <= v[i].second; j++)
      freq[j]++, cells[j].push_back(i);
  }
  for (int i = 0; i < 5005; i++) {
    if (freq[i]) ans++;
    if (freq[i] == 1) anabas[cells[i][0]]++;
    if (freq[i] == 2)
      etneen[cells[i][0]][cells[i][1]]++, etneen[cells[i][1]][cells[i][0]]++;
  }
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      int cnt = anabas[i] + anabas[j];
      cnt += etneen[i][j];
      rem = min(rem, cnt);
    }
  }
  cout << ans - rem;
  return 0;
}
