#include <bits/stdc++.h>
using namespace std;
int main() {
  int cnt[5010], vis[5010], l[5010], r[5010], i, j, n, q, maxn;
  scanf("%d%d", &n, &q);
  maxn = 0;
  for (i = 1; i <= q; i++) {
    scanf("%d %d", &l[i], &r[i]);
    for (j = l[i]; j <= r[i]; j++) {
      cnt[j]++;
    }
  }
  for (i = 1; i <= q; i++) {
    for (j = l[i]; j <= r[i]; j++) {
      cnt[j]--;
    }
    int sum = 0;
    for (j = 1; j <= n; j++) {
      if (cnt[j] == 1) {
        vis[j] = vis[j - 1] + 1;
      } else {
        vis[j] = vis[j - 1];
      }
      if (cnt[j] >= 1) {
        sum++;
      }
    }
    for (j = 1; j <= q; j++) {
      if (i == j) continue;
      maxn = max(maxn, sum - (vis[r[j]] - vis[l[j] - 1]));
    }
    for (j = l[i]; j <= r[i]; j++) {
      cnt[j]++;
    }
  }
  printf("%d\n", maxn);
  return 0;
}
