/* -*- coding: utf-8 -*-
 *
 * 3262.cc:  No.3262 豌ｴ濶ｲ繧ｳ繝ｼ繝€繝ｼ縺輔ｓ縲√◎縺ｮ蝠城｡慧蝠城｡後〒縺吶ｈ?(1<=d<=N) - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 8;

/* typedef */

/* global variables */

int ls[MAX_N], rs[MAX_N];
int ps[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d%d", ls + i, rs + i);

  for (int i = 0; i < n; i++) ps[i] = i;
  int cnt = 0;

  do {
    bool ok = true;
    for (int i = 0, px = 0; i < n; i++) {
      if (px > rs[ps[i]]) { ok = false; break; }
      px = max(px, ls[ps[i]]);
    }
    if (ok) cnt++;
  } while (next_permutation(ps, ps + n));

  printf("%d\n", cnt);
  
  return 0;
}