#include <bits/stdc++.h>
using namespace std;
int n, m;
pair<int, int> A[5000 + 2];
int DD[5000 + 2];
int DDD[5000 + 2];
void solve() {
  cin >> n >> m;
  for (int i = 1; i <= m; ++i) {
    cin >> A[i].first >> A[i].second;
    DD[A[i].first]++;
    DD[A[i].second + 1]--;
  }
  for (int i = 1; i <= n; ++i) {
    DD[i] += DD[i - 1];
  }
  int ans = -INT_MAX;
  for (int i = 1; i <= m; ++i) {
    int cnt = 0;
    for (int j = 1; j <= n; ++j) {
      if (j >= A[i].first && j <= A[i].second) {
        --DD[j];
      }
      if (DD[j]) ++cnt;
      DDD[j] = DDD[j - 1] + (DD[j] == 1);
    }
    int lose = INT_MAX;
    for (int j = i + 1; j <= m; ++j) {
      lose = min(lose, DDD[A[j].second] - DDD[A[j].first - 1]);
    }
    ans = max(ans, cnt - lose);
    for (int j = 1; j <= n; ++j) {
      if (j >= A[i].first && j <= A[i].second) {
        ++DD[j];
      }
      DDD[j] = 0;
    }
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  ;
  solve();
  return 0;
}
