#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n, q;
  cin >> n >> q;
  vector<pair<long long, long long>> intervals(q);
  vector<long long> cnt(n);
  for (long long j = 0; j < q; j++) {
    cin >> intervals[j].first >> intervals[j].second;
    intervals[j].first--;
    intervals[j].second--;
    for (long long i = intervals[j].first; i <= intervals[j].second; ++i) {
      ++cnt[i];
    }
  }
  long long answer = 0;
  for (long long A = 0; A < q; ++A) {
    long long count_positive = 0;
    vector<long long> ones(n);
    for (long long i = intervals[A].first; i <= intervals[A].second; ++i) {
      --cnt[i];
    }
    for (long long i = 0; i < n; ++i) {
      if (cnt[i] > 0) {
        ++count_positive;
      }
      if (cnt[i] == 1) {
        ++ones[i];
      }
    }
    for (long long i = 1; i < n; ++i) {
      ones[i] += ones[i - 1];
    }
    for (long long B = A + 1; B < q; ++B) {
      long long lost_ones =
          ones[intervals[B].second] -
          (intervals[B].first ? ones[intervals[B].first - 1] : 0);
      answer = max(answer, count_positive - lost_ones);
    }
    for (long long i = intervals[A].first; i <= intervals[A].second; ++i) {
      ++cnt[i];
    }
  }
  cout << answer << endl;
}
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  solve();
  return 0;
}
