#include <bits/stdc++.h>
using namespace std;
const int kMaxN = 5010;
int mn[kMaxN][2];
int val[kMaxN];
int main(void) {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, q;
  cin >> n >> q;
  vector<int> el(n + 1, 0);
  vector<pair<int, int>> pairs;
  for (int i = 0; i < q; i++) {
    int l, r;
    cin >> l >> r;
    l--;
    r--;
    pairs.emplace_back(l, r);
    el[l]++;
    el[r + 1]--;
  }
  int sum = 0;
  int cnt = 0;
  for (int i = 0; i < n; i++) {
    sum += el[i];
    val[i] = sum;
    if (sum > 0) {
      cnt++;
    }
  }
  for (int i = 0; i < n; i++) {
    mn[i][0] = val[i] == 1 ? 1 : 0;
    mn[i][1] = val[i] == 2 ? 1 : 0;
    if (i > 0) {
      mn[i][0] += mn[i - 1][0];
      mn[i][1] += mn[i - 1][1];
    }
  }
  auto getter = [&](int ind) {
    return [ind](int l, int r) {
      if (l > r) {
        return 0;
      }
      auto val = mn[r][ind - 1] - (l > 0 ? mn[l - 1][ind - 1] : 0);
      return val;
    };
  };
  auto gt_1 = getter(1);
  auto gt_2 = getter(2);
  int ans = 0;
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      int l1 = pairs[i].first;
      int r1 = pairs[i].second;
      int l2 = pairs[j].first;
      int r2 = pairs[j].second;
      if (l1 > l2) {
        swap(l1, l2);
        swap(r1, r2);
      }
      int new_cnt = cnt;
      if (l2 > r1) {
        new_cnt -= gt_1(l1, r1);
        new_cnt -= gt_1(l2, r2);
      } else {
        new_cnt -= gt_1(l1, l2 - 1);
        new_cnt -= gt_2(l2, min(r1, r2));
        new_cnt -= gt_1(min(r1, r2) + 1, max(r1, r2));
      }
      ans = max(ans, new_cnt);
    }
  }
  cout << ans << endl;
}
