#if __has_include(<yoniha/all.h>)
#include <yoniha/all.h>
using namespace atcoder;
#else
#include <bits/stdc++.h>
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
#endif
#endif
using namespace std;

#define int long long
#define all(x) (x).begin(), (x).end()
#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--)
template <typename T> bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template <typename T> bool chmin(T &a,const T &b){if(a>b){a=b;return true;}return false;}

// using mint = modint;

signed main(){
  int n; cin >> n;
  vector<pair<int, int>> lr(n);
  for(auto&& [l, r] : lr) cin >> l >> r;
  vector<int> per(n); rep(i, n) per.at(i) = i;
  int ans = 0;
  do{
    bool ok = true;
    int before = -1;
    rep(i, n){
      auto [l, r] = lr.at(per.at(i));
      ok &= r >= before;
      before = max(l, before);
    }
    ans += ok;
  }while(next_permutation(all(per)));
  cout << ans << '\n';
}