#include <bits/stdc++.h>
using namespace std;
bool vis[10000000];
const int MAXN = 1e4 + 10;
vector<int> ve;
struct point {
  int l, r;
} arr[MAXN];
int point[MAXN];
int per[MAXN];
int main() {
  int n, q;
  cin >> n >> q;
  for (int i = 1; i <= q; i++) {
    cin >> arr[i].l;
    cin >> arr[i].r;
    for (int j = arr[i].l; j <= arr[i].r; j++) point[j]++;
  }
  int ans = 0;
  for (int i = 1; i <= q; i++) {
    int sum = 0;
    memset(per, 0, sizeof(per));
    for (int j = arr[i].l; j <= arr[i].r; j++) point[j]--;
    for (int j = 1; j <= n; j++) {
      if (point[j] == 1) {
        per[j] = per[j - 1] + point[j];
      } else
        per[j] = per[j - 1];
      if (point[j]) sum++;
    }
    for (int j = i + 1; j <= q; j++) {
      ans = max(ans, sum - (per[arr[j].r] - per[arr[j].l - 1]));
    }
    for (int j = arr[i].l; j <= arr[i].r; j++) point[j]++;
  }
  cout << ans << endl;
  return 0;
}
