#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e9 + 7;
const int N = 5e3 + 10;
int flag[N];
int sum1[N];
int sum2[N];
pair<int, int> p[N];
pair<int, int> combine(int l1, int r1, int l2, int r2) {
  return make_pair(max(l1, l2), min(r1, r2));
}
pair<int, int> sub(int l1, int r1, int l2, int r2) {
  if (l1 == l2) {
    return {r2 + 1, r1};
  }
  if (r1 == r2) {
    return {l1, l2 - 1};
  }
  return {l1, r1};
}
int getans(pair<int, int> p, int sum[]) {
  int l, r;
  tie(l, r) = p;
  if (l > r) return 0;
  return sum[r] - sum[l - 1];
}
int main() {
  int n, k;
  cin >> n >> k;
  for (int i = 1; i <= k; i++) {
    int l, r;
    cin >> l >> r;
    p[i] = {l, r};
    flag[l]++;
    flag[r + 1]--;
  }
  for (int i = 1; i <= n; i++) flag[i] += flag[i - 1];
  int tot = 0;
  for (int i = 1; i <= n; i++) {
    sum1[i] = sum1[i - 1] + (flag[i] == 1);
    sum2[i] = sum2[i - 1] + (flag[i] == 2);
    tot += flag[i] > 0;
  }
  int ans = 0;
  for (int i = 1; i <= k; i++) {
    for (int j = i + 1; j <= k; j++) {
      int l1, r1;
      int l2, r2;
      tie(l1, r1) = p[i];
      tie(l2, r2) = p[j];
      auto pp = combine(l1, r1, l2, r2);
      if (pp.first > pp.second) pp = {0, -1};
      int cnt = getans(pp, sum2);
      auto p1 = sub(l1, r1, pp.first, pp.second);
      auto p2 = sub(l2, r2, pp.first, pp.second);
      cnt += getans(p1, sum1) + getans(p2, sum1);
      ans = max(ans, tot - cnt);
    }
  }
  printf("%d\n", ans);
  return 0;
}
