#include <bits/stdc++.h>
using namespace std;
void fast() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
}
const int N = 5005;
int cnt[N], pre[N], pre2[N];
int l[N], r[N];
pair<int, int> get(pair<int, int> a, pair<int, int> b) {
  if (a > b) swap(a, b);
  if (a.second < b.first) return {-1, -1};
  return {max(a.first, b.first), min(a.second, b.second)};
}
int main() {
  fast();
  int n, q;
  cin >> n >> q;
  for (int i = 0; i < q; i++) {
    cin >> l[i] >> r[i];
    cnt[l[i]]++;
    cnt[r[i] + 1]--;
  }
  for (int i = 1; i <= n; i++) {
    cnt[i] += cnt[i - 1];
  }
  int sum = 0;
  for (int i = 1; i <= n; i++) {
    pre[i] = pre[i - 1] + (cnt[i] == 1);
    pre2[i] = pre2[i - 1] + (cnt[i] == 2);
    sum += (cnt[i] > 0);
  }
  int ans = 0;
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      int tmp = sum;
      tmp -= (pre[r[i]] - pre[l[i] - 1]);
      tmp -= (pre[r[j]] - pre[l[j] - 1]);
      pair<int, int> common = get({l[i], r[i]}, {l[j], r[j]});
      if (common.first != -1) {
        tmp -= (pre2[common.second] - pre2[common.first - 1]);
      }
      ans = max(ans, tmp);
    }
  }
  cout << ans << endl;
  return 0;
}
