#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  vector<long long> paint(5010), ones(5010), twos(5010);
  long long n, q;
  cin >> n >> q;
  vector<pair<long long, long long>> A(q);
  for (long long i = 0; i < q; i++) {
    long long l, r;
    cin >> l >> r;
    A[i].first = l;
    A[i].second = r;
    paint[l] += 1;
    paint[r + 1] -= 1;
  }
  for (long long i = 1; i < 5010; i++) {
    paint[i] += paint[i - 1];
  }
  for (long long i = 1; i < 5010; i++) {
    ones[i] += paint[i] == 1;
    twos[i] += paint[i] == 2;
  }
  for (long long i = 1; i < 5010; i++) ones[i] += ones[i - 1];
  for (long long i = 1; i < 5010; i++) twos[i] += twos[i - 1];
  long long total = 0, mx = 0;
  for (long long i = 0; i < 5010; i++) {
    if (paint[i]) total++;
  }
  for (long long i = 0; i < q; i++) {
    for (long long j = i + 1; j < q; j++) {
      long long a_only = ones[A[i].second] - ones[A[i].first - 1];
      long long b_only = ones[A[j].second] - ones[A[j].first - 1];
      long long l = max(A[i].first, A[j].first);
      long long r = min(A[i].second, A[j].second);
      long long ab = (l <= r ? twos[r] - twos[l - 1] : 0);
      mx = max(mx, total - (a_only + b_only + ab));
    }
  }
  cout << mx << '\n';
}
