#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, q;
  cin >> n >> q;
  pair<int, pair<int, int>> r[2 * q];
  pair<int, int> w[q];
  int a[n];
  int p[q];
  fill(p, p + q, 0);
  fill(a, a + n, 0);
  for (int i = 0; i < q; i++) {
    cin >> w[i].first >> w[i].second;
    for (int j = w[i].first - 1; j <= w[i].second - 1; j++) {
      a[j]++;
    }
  }
  for (int i = 0; i < q; i++) {
    for (int j = w[i].first - 1; j <= w[i].second - 1; j++) {
      if (a[j] == 1) p[i]++;
    }
  }
  int mn = n;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] > 0) ans++;
  }
  int pr[n + 1];
  fill(pr, pr + (n + 1), 0);
  for (int i = 1; i <= n; i++) {
    pr[i] = pr[i - 1];
    if (a[i - 1] == 2) pr[i]++;
  }
  for (int j1 = 0; j1 < q; j1++) {
    for (int j2 = j1 + 1; j2 < q; j2++) {
      if (j1 == j2) continue;
      int t = p[j1] + p[j2];
      int a = w[j1].first, b = w[j1].second, c = w[j2].first, d = w[j2].second;
      if (a > d || c > b) {
      } else if (a < c) {
        if (b < d) {
          t += pr[b] - pr[c - 1];
        } else {
          t += pr[d] - pr[c - 1];
        }
      } else {
        if (b > d)
          t += pr[d] - pr[a - 1];
        else
          t += pr[b] - pr[a - 1];
      }
      mn = min(t, mn);
    }
  }
  cout << ans - mn;
  return 0;
}
