#include <bits/stdc++.h>
using namespace std;
bool func(pair<int, int> &a, pair<int, int> &b) { return a.first < b.first; }
int main() {
  int n, q;
  vector<pair<int, int>> arr;
  int input1, input2;
  int i, j;
  int level[5001] = {
      0,
  };
  priority_queue<int, vector<int>, greater<int>> exit;
  int curlev = 1;
  int cur, prev = 0;
  int acc1[5001], acc2[5001];
  pair<int, int> interv1, interv2, temp;
  int over1, over2;
  int last = 0, sum;
  int cand;
  int lmax = 0;
  scanf("%d %d", &n, &q);
  for (i = 0; i < q; i++) {
    scanf("%d %d", &input1, &input2);
    arr.push_back(pair<int, int>(input1, input2));
    for (j = input1; j <= input2; j++) {
      level[j]++;
    }
  }
  sort(arr.begin(), arr.end(), func);
  sum = n;
  for (i = 0; i < q; i++) {
    if (arr[i].first > prev + 1) {
      sum -= arr[i].first - (prev + 1);
    }
    if (arr[i].second > prev) {
      prev = arr[i].second;
    }
  }
  sum -= n - prev;
  acc1[0] = acc2[0] = 0;
  for (i = 1; i <= n; i++) {
    acc1[i] = acc1[i - 1] + (level[i] == 1);
    acc2[i] = acc2[i - 1] + (level[i] == 2);
  }
  for (i = 0; i < q - 1; i++) {
    for (j = i + 1; j < q; j++) {
      interv1 = arr[i];
      interv2 = arr[j];
      over1 = 1;
      over2 = 0;
      if (interv2.first <= interv1.second) {
        over1 = interv2.first;
        if (interv2.second < interv1.second) {
          over2 = interv2.second;
        } else {
          over2 = interv1.second;
        }
      }
      cand = sum -
             ((acc1[interv1.second] - acc1[interv1.first - 1]) +
              (acc1[interv2.second] - acc1[interv2.first - 1])) -
             (acc2[over2] - acc2[over1 - 1]);
      if (cand > lmax) {
        lmax = cand;
      }
    }
  }
  printf("%d\n", lmax);
  return 0;
}
