#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<P> lr(n);
  for (auto& [l, r] : lr) cin >> l >> r;
  VI ord(n);
  iota(all(ord), 0);
  int ans = 0;
  do {
    int now = 0;
    for (int i : ord) {
      auto [l, r] = lr[i];
      if (now > r) {
        now = -1;
        break;
      }
      chmax(now, l);
    }
    ans += now != -1;
  } while (next_permutation(all(ord)));
  cout << ans << '\n';
}