#include <bits/stdc++.h>
using namespace std;
const long long N = 5000;
long long cumulative[N + 10], arr[N + 10][3];
signed main() {
  ios ::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, q;
  cin >> n >> q;
  vector<pair<long long, long long> > pi;
  for (long long i = 0; i < q; i++) {
    long long a, b;
    cin >> a >> b;
    cumulative[a] += 1;
    cumulative[b + 1] -= 1;
    pi.push_back({a, b});
  }
  for (long long i = 1; i <= n; i++) {
    cumulative[i] += cumulative[i - 1];
  }
  for (long long i = 1; i <= n; i++) {
    for (long long j = 0; j < 3; j++) {
      if (cumulative[i] == j) {
        arr[i][j] += (arr[i - 1][j] + 1);
      } else {
        arr[i][j] += arr[i - 1][j];
      }
    }
  }
  long long sz = pi.size();
  sort(pi.begin(), pi.end());
  long long ans = 0;
  for (long long i = 0; i < sz - 1; i++) {
    long long x = pi[i].first;
    long long y = pi[i].second;
    for (long long j = i + 1; j < sz; j++) {
      long long two = 0;
      long long a = pi[j].first;
      long long b = pi[j].second;
      if (a <= y) {
        long long ax = max(x, a);
        long long by = min(y, b);
        two = (arr[by][2] - arr[ax - 1][2]);
      }
      long long zero = (arr[n][0] - arr[0][0]);
      long long one = (arr[y][1] - arr[x - 1][1]) + (arr[b][1] - arr[a - 1][1]);
      long long total = one + two + zero;
      long long res = n - total;
      ans = max(ans, res);
    }
  }
  cout << ans << endl;
  return 0;
}
