#include <bits/stdc++.h>
using namespace std;
int n, q, cnt[5005], fr1[5005], fr2[5005];
pair<int, int> seg[5005], tmp[5005];
void Read() {
  scanf("%d%d", &n, &q);
  for (int i = 1; i <= q; i++) {
    scanf("%d%d", &seg[i].first, &seg[i].second);
  }
}
void Solve() {
  int sum = 0;
  for (int i = 1; i <= q; i++) {
    for (int j = seg[i].first; j <= seg[i].second; j++) {
      cnt[j]++;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (cnt[i]) {
      sum++;
    }
    if (cnt[i] == 1) {
      fr1[i] = fr1[i - 1] + 1;
    } else {
      fr1[i] = fr1[i - 1];
    }
    if (cnt[i] == 2) {
      fr2[i] = fr2[i - 1] + 1;
    } else {
      fr2[i] = fr2[i - 1];
    }
  }
  int ans = 0;
  for (int i = 1; i <= q; i++) {
    for (int j = i + 1; j <= q; j++) {
      if (seg[i].second < seg[j].first) {
        ans = max(ans, sum - (fr1[seg[i].second] - fr1[seg[i].first - 1]) -
                           (fr1[seg[j].second] - fr1[seg[j].first - 1]));
      } else {
        if (seg[j].second <= seg[i].second) {
          ans = max(ans, sum - (fr1[seg[i].second] - fr1[seg[i].first - 1]) -
                             (fr2[seg[j].second] - fr2[seg[j].first - 1]));
        } else {
          ans = max(ans, sum - (fr1[seg[j].first - 1] - fr1[seg[i].first - 1]) -
                             (fr2[seg[i].second] - fr2[seg[j].first - 1]) -
                             fr1[seg[j].second] + fr1[seg[i].second]);
        }
      }
    }
  }
  printf("%d", ans);
}
int main() {
  Read();
  sort(seg + 1, seg + q + 1);
  Solve();
  return 0;
}
