#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5005;
int amt[MAXN], swp[MAXN], ct1[MAXN], ct2[MAXN];
int main() {
  int n, q;
  cin >> n >> q;
  vector<pair<int, int> > pairs(q);
  for (int i = 0; i < q; i++) {
    int l, r;
    cin >> l >> r;
    swp[l]++;
    swp[r + 1]--;
    pairs[i] = pair<int, int>{l, r};
  }
  int cu = 0, ini = 0;
  for (int i = 1; i <= n; i++) {
    cu += swp[i];
    amt[i] = cu;
  }
  int ans = 0;
  for (int i = 0; i < q; i++) {
    auto p1 = pairs[i];
    ini = 0;
    for (int x = p1.first; x <= p1.second; x++) amt[x]--;
    ct1[0] = 0;
    for (int x = 1; x <= n; x++) {
      ct1[x] = ct1[x - 1];
      if (amt[x] == 1) ct1[x]++;
      if (amt[x] > 0) ini++;
    }
    for (int j = 0; j < q; j++) {
      if (j == i) continue;
      auto p2 = pairs[j];
      int ex = ct1[p2.second] - ct1[p2.first - 1];
      ans = max(ans, ini - ex);
    }
    for (int x = p1.first; x <= p1.second; x++) amt[x]++;
  }
  cout << ans << endl;
}
