#include <bits/stdc++.h>
using namespace std;
int n, q, a[5005][2], g[5005][5005], s1, s2, ans = 1e9;
int main() {
  scanf("%d%d", &n, &q);
  for (int i = 1; i <= q; i++) scanf("%d%d", &a[i][0], &a[i][1]);
  for (int i = 1; i <= n; i++) {
    s1 = s2 = 0;
    bool flg = 1;
    for (int j = 1; j <= q; j++) {
      if (a[j][0] > i || a[j][1] < i) continue;
      if (!s1)
        s1 = j;
      else if (!s2)
        s2 = j;
      else {
        flg = 0;
        break;
      }
    }
    if (flg) g[s1][s2]++;
  }
  for (int i = 1; i <= q; i++)
    for (int j = i + 1; j <= q; j++)
      ans = min(ans, g[0][0] + g[i][0] + g[j][0] + g[i][j]);
  printf("%d", n - ans);
  return 0;
}
