#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
priority_queue<int> answers;
int fence[5005];
int paint_1[5005];
int paint_2[5005];
int total;
pair<int, int> painters[5005];
int main() {
  int n, q;
  cin >> n >> q;
  for (int i = 0; i < q; i++) {
    cin >> painters[i].first >> painters[i].second;
    for (int j = painters[i].first; j <= painters[i].second; j++) {
      fence[j]++;
      if (fence[j] == 1) total++;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (fence[i] == 1)
      paint_1[i] = paint_1[i - 1] + 1;
    else
      paint_1[i] = paint_1[i - 1];
    if (fence[i] == 2)
      paint_2[i] = paint_2[i - 1] + 1;
    else
      paint_2[i] = paint_2[i - 1];
  }
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      int lower = max(painters[i].first, painters[j].first);
      int upper = min(painters[i].second, painters[j].second);
      int answer = total;
      answer = answer -
               (paint_1[painters[i].second] - paint_1[painters[i].first - 1]) -
               (paint_1[painters[j].second] - paint_1[painters[j].first - 1]);
      if (lower <= upper) {
        answer = answer + (paint_1[upper] - paint_1[lower - 1]) -
                 (paint_2[upper] - paint_2[lower - 1]);
      }
      answers.push(answer);
    }
  }
  cout << answers.top() << endl;
  return 0;
}
