#include <bits/stdc++.h>
using namespace std;
const int N = 5005;
int a[N], sum1[N], sum2[N];
struct Node {
  int l, r;
} q[N];
inline int Max(int a, int b) { return a > b ? a : b; }
int main() {
  int n, m, sum = 0;
  cin >> n >> m;
  for (int i = 1; i <= m; i++)
    cin >> q[i].l >> q[i].r, a[q[i].l]++, a[q[i].r + 1]--;
  for (int i = 1; i <= n; i++) a[i] += a[i - 1], sum += (a[i] != 0);
  for (int i = 1; i <= n; i++)
    sum1[i] = sum1[i - 1] + (a[i] == 1), sum2[i] = sum2[i - 1] + (a[i] == 2);
  int ans = 0;
  for (int i = 1; i <= m; i++)
    for (int j = i + 1; j <= m; j++) {
      int l1 = q[i].l, r1 = q[i].r, l2 = q[j].l, r2 = q[j].r;
      if (l1 > l2) swap(l1, l2), swap(r1, r2);
      if (l1 <= l2 && l2 <= r1 && r1 <= r2) {
        int cnt = sum1[r2] - sum1[l1 - 1] + sum2[r1] - sum2[l2 - 1];
        ans = Max(ans, sum - cnt);
      } else if (l1 <= l2 && l2 <= r2 && r2 <= r1) {
        int cnt = sum1[r1] - sum1[l1 - 1] + sum2[r2] - sum2[l2 - 1];
        ans = Max(ans, sum - cnt);
      } else {
        int cnt = sum1[r1] - sum1[l1 - 1] + sum1[r2] - sum1[l2 - 1];
        ans = Max(ans, sum - cnt);
      }
    }
  printf("%d", ans);
  return 0;
}
