#include <bits/stdc++.h>
using namespace std;
long long b[5005], c[5005], d[5005], e[5005];
int main() {
  long long n, q;
  cin >> n >> q;
  vector<pair<long long, long long> > a;
  for (int i = 0; i < q; i++) {
    long long u, v;
    cin >> u >> v;
    a.push_back(make_pair(u, v));
    b[v + 1] += -1;
    b[u] += 1;
  }
  for (int i = 1; i <= n; i++) {
    b[i] += b[i - 1];
  }
  for (int i = 1; i <= n; i++) {
    if (b[i] == 1LL) {
      d[i] = d[i - 1] + 1;
    } else {
      d[i] = d[i - 1];
    }
    if (b[i] == 2LL || b[i] == 1LL) {
      e[i] = e[i - 1] + 1;
    } else {
      e[i] = e[i - 1];
    }
  }
  long long h = 0;
  for (int i = 1; i <= n; i++) {
    if (b[i] == .0) {
      h++;
    }
  }
  long long fans = 0LL;
  for (int i = 0; i < q - 1; i++) {
    for (int j = i + 1; j < q; j++) {
      long long ans = n - h;
      long long l1, r1, l2, r2;
      l1 = a[i].first;
      r1 = a[i].second;
      l2 = a[j].first;
      r2 = a[j].second;
      if (r1 < l2 || r2 < l1) {
        ans = ans - (d[r1] - d[l1 - 1]);
        ans = ans - (d[r2] - d[l2 - 1]);
      } else {
        long long fl1 = min(l1, l2), fl2 = max(l1, l2), fr1 = min(r1, r2),
                  fr2 = max(r1, r2);
        ans = ans - (e[fr1] - e[fl2 - 1]);
        fr1++;
        fl2--;
        if (fr1 <= fr2) {
          ans = ans - (d[fr2] - d[fr1 - 1]);
        }
        if (fl1 <= fl2) {
          ans = ans - (d[fl2] - d[fl1 - 1]);
        }
      }
      fans = max(fans, ans);
    }
  }
  cout << fans << "\n";
}
