#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, q;
  cin >> n >> q;
  long long x, y;
  vector<pair<int, int>> v;
  vector<long long> a(n + 2, 0), b(n + 2, 0), cc(n + 2, 0);
  for (long long i = 0; i < q; ++i) {
    cin >> x >> y;
    v.push_back(make_pair(x, y));
    a[x] += 1;
    a[y + 1] -= 1;
  }
  for (long long i = 1; i < n + 1; ++i) {
    a[i] += a[i - 1];
  }
  long long sum = 0;
  for (long long i = 1; i < n + 1; ++i) {
    if (a[i] != 0) sum++;
  }
  for (long long i = 1; i < n + 1; ++i) {
    b[i] = b[i - 1];
    cc[i] = cc[i - 1];
    if (a[i] == 1) b[i]++;
    if (a[i] == 2) cc[i]++;
  }
  long long l1, l2, r1, r2;
  sort(v.begin(), v.end());
  long long ans = INT_MIN;
  for (long long i = 0; i < q; ++i) {
    l1 = v[i].first;
    r1 = v[i].second;
    for (long long j = i + 1; j < q; ++j) {
      l2 = v[j].first;
      r2 = v[j].second;
      if (r1 < l2) {
        ans = max(ans, sum - (b[r1] - b[l1 - 1]) - (b[r2] - b[l2 - 1]));
      } else {
        if (r2 < r1)
          ans = max(ans, sum - (b[l2 - 1] - b[l1 - 1]) - (b[r1] - b[r2]) -
                             (cc[r2] - cc[l2 - 1]));
        else {
          ans = max(ans, sum - (b[l2 - 1] - b[l1 - 1]) - (b[r2] - b[r1]) -
                             (cc[r1] - cc[l2 - 1]));
        }
      }
    }
  }
  cout << ans;
  return 0;
}
