#include <bits/stdc++.h>
using namespace std;
const int N = 5e3 + 5;
bool done[N];
int a[N];
int b[N][N];
vector<int> vec[N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, q;
  cin >> n >> q;
  for (int i = 1, l, r; i <= q; i++) {
    cin >> l >> r;
    for (int j = l; j <= r; j++) {
      if (done[j]) continue;
      vec[j].push_back(i);
      if (vec[j].size() > 2) {
        done[j] = 1;
        vec[j].clear();
      }
    }
  }
  int total = 0;
  for (int i = 1; i <= n; i++) {
    if (done[i]) {
      total++;
      continue;
    }
    if (vec[i].size() == 1) {
      a[vec[i][0]]++;
      total++;
    }
    if (vec[i].size() == 2) {
      int x = vec[i][0], y = vec[i][1];
      if (x > y) swap(x, y);
      b[x][y]++;
      total++;
    }
  }
  int ans = n;
  for (int i = 1; i <= q; i++) {
    for (int j = i + 1; j <= q; j++) {
      ans = min(ans, a[i] + a[j] + b[i][j]);
    }
  }
  cout << total - ans << endl;
  return 0;
}
