#include <bits/stdc++.h>
using namespace std;
int solve() {
  int n, q;
  cin >> n >> q;
  int l[q], r[q], zero = 0;
  int a[n + 1];
  memset(a, 0, sizeof(a));
  for (int i = 0; i < q; i++) {
    cin >> l[i] >> r[i];
    a[l[i]]++;
    if (r[i] != n) {
      a[r[i] + 1]--;
    }
  }
  for (int i = 0; i < n + 1; i++) {
    if (i != 0) {
      a[i] += a[i - 1];
    }
  }
  int c = 0, cnt1[n + 1], cnt2[n + 1];
  memset(cnt1, 0, sizeof(cnt1));
  memset(cnt2, 0, sizeof(cnt2));
  for (int i = 1; i < n + 1; i++) {
    if (a[i] == 0) zero++;
    cnt1[i] = cnt1[i - 1];
    cnt2[i] = cnt2[i - 1];
    if (a[i] == 1) {
      cnt1[i] += 1;
    } else if (a[i] == 2) {
      cnt2[i] += 1;
    }
  }
  long long int ansM = 0;
  for (int i = 0; i < q - 1; i++) {
    for (int j = i + 1; j < q; j++) {
      long long int ans = zero;
      if (l[i] != 1) {
        ans += cnt1[r[i]] - cnt1[l[i] - 1];
      } else
        ans += cnt1[r[i]];
      if (l[j] != 1) {
        ans += cnt1[r[j]] - cnt1[l[j] - 1];
      } else {
        ans += cnt1[r[j]];
      }
      int l1 = max(l[i], l[j]), r1 = min(r[i], r[j]);
      if (l1 <= r1) {
        if (l1 != 1) {
          ans += cnt2[r1] - cnt2[l1 - 1];
        } else {
          ans += cnt2[r1];
        }
      }
      ansM = max(ansM, n - ans);
    }
  }
  cout << ansM;
  return 0;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t;
  t = 1;
  while (t--) {
    solve();
  }
}
