#include <bits/stdc++.h>
using namespace std;
const int maxx = 5010;
int pre[maxx + 10];
vector<pair<int, int> > v;
vector<int> one, two;
int main() {
  int n, q, kk = 0;
  cin >> n >> q;
  for (int i = 1; i <= q; i++) {
    int l, r;
    cin >> l >> r;
    pre[l]++;
    pre[r + 1]--;
    v.push_back({l, r});
  }
  for (int i = 1; i <= n; i++) {
    pre[i] = pre[i] + pre[i - 1];
    if (pre[i] == 1) {
      one.push_back(i);
    } else if (pre[i] == 2) {
      two.push_back(i);
    } else if (pre[i] == 0) {
      kk++;
    }
  }
  int ans = 1e9 + 10;
  for (int i = 0; i < v.size(); i++) {
    for (int j = i + 1; j < v.size(); j++) {
      pair<int, int> a = v[i], b = v[j];
      int sum = 0;
      auto lt = lower_bound(one.begin(), one.end(), a.first);
      auto rt = upper_bound(one.begin(), one.end(), a.second);
      if (lt != one.end() && *lt <= a.second) {
        rt--;
        int x = lt - one.begin(), y = rt - one.begin();
        sum += max(y - x + 1, 0);
      }
      lt = lower_bound(one.begin(), one.end(), b.first);
      rt = upper_bound(one.begin(), one.end(), b.second);
      if (lt != one.end() && *lt <= b.second) {
        rt--;
        int x = lt - one.begin(), y = rt - one.begin();
        sum += max(y - x + 1, 0);
      }
      int r = min(a.second, b.second), l = max(a.first, b.first);
      if (l <= r) {
        lt = lower_bound(two.begin(), two.end(), l);
        rt = upper_bound(two.begin(), two.end(), r);
        if (lt != two.end() && *lt <= r) {
          rt--;
          int x = lt - two.begin(), y = rt - two.begin();
          sum += max(y - x + 1, 0);
        }
      }
      ans = min(ans, sum);
    }
  }
  cout << n - (ans + kk) << endl;
  return 0;
}
