#include <bits/stdc++.h>
using namespace std;
const long long N = 5001;
long long sum1[N], sum2[N];
long long a[N];
void print(pair<long long, long long> a) {
  cout << a.first << " " << a.second << endl;
}
pair<long long, long long> inter(pair<long long, long long> a,
                                 pair<long long, long long> b) {
  pair<long long, long long> p;
  if (a.second < b.first) {
    p = {-1, -1};
  } else {
    p = {b.first, min(a.second, b.second)};
  }
  return p;
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n, q;
  cin >> n >> q;
  long long sum = 0;
  vector<pair<long long, long long> > v;
  for (long long i = 0; i < q; ++i) {
    long long l, r;
    cin >> l >> r;
    v.push_back({l, r});
    for (long long j = l; j <= r; ++j) a[j]++;
  }
  for (long long i = 1; i <= n; ++i) {
    sum1[i] = sum1[i - 1] + (a[i] == 1);
    sum2[i] = sum2[i - 1] + (a[i] == 2);
    sum += (bool)a[i];
  }
  sort(v.begin(), v.end());
  long long ans = 0;
  for (long long i = 0; i < q; ++i) {
    for (long long j = i + 1; j < q; ++j) {
      pair<long long, long long> p = inter(v[i], v[j]);
      long long cnt = sum1[v[i].second] - sum1[v[i].first - 1];
      cnt += sum1[v[j].second] - sum1[v[j].first - 1];
      if (p.first != -1) {
        cnt += sum2[p.second] - sum2[p.first - 1];
      }
      ans = max(ans, sum - cnt);
    }
  }
  cout << ans;
  return 0;
}
