#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
pair<int, int> p[5005];
int arr[5005] = {0};
int brr[5005][4];
int main() {
  ios ::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, q;
  cin >> n >> q;
  int l, r;
  for (int i = 0; i < q; i++) {
    cin >> l >> r;
    arr[l]++;
    arr[r + 1]--;
    p[i] = {l, r};
  }
  sort(p, p + q);
  for (int i = 1; i <= n; i++) {
    arr[i] = arr[i - 1] + arr[i];
  }
  int sum = 0;
  for (int i = 0; i <= n; i++) {
    sum += int(arr[i] != 0);
    int x = arr[i];
    if (x >= 3) x = 3;
    brr[i][x] = 1;
  }
  for (int i = 0; i <= n; i++) {
    for (int j = 0; j < 4; j++) {
      brr[i][j] += brr[i - 1][j];
    }
  }
  int ans = 0;
  int tem = 0;
  int l1, r1;
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      int l1, r1, l2, r2;
      l1 = p[i].first;
      r1 = p[i].second;
      l2 = p[j].first;
      r2 = p[j].second;
      if (r1 < l2) {
        tem = brr[r1][1] - brr[l1 - 1][1] + brr[r2][1] - brr[l2 - 1][1];
        tem = sum - tem;
        ans = max(ans, tem);
        continue;
      }
      if (r2 > r1) swap(r1, r2);
      tem = brr[r1][1] - brr[l1 - 1][1] + brr[r2][2] - brr[l2 - 1][2];
      tem = sum - tem;
      ans = max(tem, ans);
    }
  }
  cout << ans << endl;
  return 0;
}
