#include <bits/stdc++.h>
using namespace std;
const int N = 5e3 + 5;
int n, q;
int l[N], r[N];
int a[N], cnt[N];
int cum[N][4];
int get(int l, int r, int fen) { return cum[r][fen] - cum[l - 1][fen]; }
int solve(int i, int j) {
  int l1 = l[i], l2 = l[j];
  int r1 = r[i], r2 = r[j];
  if (l1 > l2) swap(l1, l2), swap(r1, r2);
  int nl = max(l1, l2);
  int nr = min(r1, r2);
  if (nl > nr) return get(l1, r1, 1) + get(l2, r2, 1);
  return get(l1, nl - 1, 1) + get(nl, nr, 2) + get(nr + 1, max(r1, r2), 1);
}
int main() {
  scanf("%d%d", &n, &q);
  for (int i = 0; i < q; ++i) {
    scanf("%d%d", l + i, r + i);
    ++cnt[l[i]];
    --cnt[r[i] + 1];
  }
  int kolo = 0;
  for (int i = 1; i <= n; ++i) cnt[i] += cnt[i - 1], kolo += (cnt[i] > 0);
  for (int j = 1; j <= 2; ++j)
    for (int i = 1; i <= n; ++i) cum[i][j] = cum[i - 1][j] + (cnt[i] == j);
  int mn = INT_MAX;
  for (int i = 0; i < q; ++i)
    for (int j = i + 1; j < q; ++j) mn = min(mn, solve(i, j));
  printf("%d\n", kolo - mn);
}
