#include <bits/stdc++.h>
using namespace std;
int n, q;
pair<int, int> A[5000];
int total[5001];
int suffix[5001];
bool inside(pair<int, int> &p, int k) { return k >= p.first && k <= p.second; }
void solve() {
  cin >> n >> q;
  for (int i = 0; i <= q - 1; i++) {
    cin >> A[i].first >> A[i].second;
  }
  for (int i = 0; i <= q - 1; i++) {
    for (int j = A[i].first; j <= A[i].second; j++) {
      total[j]++;
    }
  }
  int ans = 0;
  for (int i = 0; i <= q - 2; i++) {
    for (int j = A[i].first; j <= A[i].second; j++) total[j]--;
    int a = 0;
    for (int j = 1; j <= n; j++)
      suffix[j] = suffix[j - 1] + (total[j] == 1), a += (!!total[j]);
    int mi = n;
    for (int j = i + 1; j <= q - 1; j++) {
      mi = min(mi, suffix[A[j].second] - suffix[A[j].first - 1]);
    }
    ans = max(ans, a - mi);
    for (int j = A[i].first; j <= A[i].second; j++) total[j]++;
  }
  cout << ans << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int T = 1;
  for (int t = 1; t <= T; t++) {
    solve();
  }
  return 0;
}
