#include <bits/stdc++.h>
using namespace std;
class Solution {
 public:
  int maxPainted(int n, vector<pair<int, int>>& work, int q) {
    vector<int> fence(n, 0);
    for (int i = 0; i < q; i++)
      for (int j = work[i].first; j <= work[i].second; j++) fence[j]++;
    vector<int> zeros(n + 1, 0), ones(n + 1, 0), twos(n + 1, 0);
    for (int i = 0; i < n; i++)
      zeros[i + 1] = zeros[i] + (fence[i] == 0),
                ones[i + 1] = ones[i] + (fence[i] == 1),
                twos[i + 1] = twos[i] + (fence[i] == 2);
    sort(work.begin(), work.end());
    int res = 0;
    for (int i = 0; i < q; i++)
      for (int j = i + 1; j < q; j++) {
        if (work[j].first > work[i].second)
          res = max(res, n - how_many(zeros, n, 0, n - 1) -
                             how_many(ones, n, work[i].first, work[i].second) -
                             how_many(ones, n, work[j].first, work[j].second));
        else if (work[j].second <= work[i].second)
          res = max(res,
                    n - how_many(zeros, n, 0, n - 1) -
                        how_many(ones, n, work[i].first, work[j].first - 1) -
                        how_many(twos, n, work[j].first, work[j].second) -
                        how_many(ones, n, work[j].second + 1, work[i].second));
        else
          res = max(res,
                    n - how_many(zeros, n, 0, n - 1) -
                        how_many(ones, n, work[i].first, work[j].first - 1) -
                        how_many(twos, n, work[j].first, work[i].second) -
                        how_many(ones, n, work[i].second + 1, work[j].second));
      }
    return res;
  }

 public:
  int how_many(const vector<int>& vec, int n, int l, int r) {
    if (l > r) return 0;
    return vec[r + 1] - vec[l];
  }
};
int main() {
  int n, q;
  cin >> n >> q;
  vector<pair<int, int>> work(q);
  for (int i = 0; i < q; i++)
    cin >> work[i].first >> work[i].second, work[i].first--, work[i].second--;
  cout << Solution().maxPainted(n, work, q) << endl;
  return 0;
}
