#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;

using isize = size_t;
using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
using i128 = __int128_t;
using u128 = __uint128_t;
using f64 = long double;
using p2 = pair<i64, i64>;
using el = tuple<i64, i64, i64>;
using mint = atcoder::modint998244353;

void _main();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(18);
  _main();
}

i64 pow(i64 x, i64 n) {
  i64 res = 1;
  i64 t = x;
  while (n > 0) {
    if (n & 1) res *= t;
    t *= t;
    n >>= 1;
  }
  return res;
}

void _main() {
  i64 n;
  cin >> n;
  vector<p2> lr(n);
  vector<i64> p(n);
  for (i64 i = 0; i < n; i++) {
    cin >> lr[i].first >> lr[i].second;
    p[i] = i;
  }
  i64 ans = 0;
  do {
    i64 now = -1e18;
    bool ok = true;
    for (i64 i = 0; i < n; i++) {
      auto [l, r] = lr[p[i]];
      now = max(now, l);
      ok &= now <= r;
    }
    ans += ok;
  } while (next_permutation(p.begin(), p.end()));
  cout << ans << "\n";
}