#include <bits/stdc++.h>
#include <atcoder/lazysegtree>
using namespace std;
using ll = long long;
const int INF = 1001001001;
const ll LINF = 3001001001001001001;
const int MOD = 998244353;
const string Yes = "Yes";
const string No = "No";
#define reps(i, a, n) for (ll i = (a); i < (ll)(n); ++i)
#define rep(i, n) reps(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {for (T &in : v)is >> in;return is;}
vector<pair<int,int>> dir = {{0,1},{1,0},{0,-1},{-1,0},};

int main() {
  int n;cin>>n;
  vector<int> l(n),r(n);
  rep(i,n) cin>>l.at(i)>>r.at(i);
  vector<int> ord(n);
  rep(i,n) ord.at(i) = i;
  int ans = 0;
  do {
    bool ok = true;
    int cur = 0;
    for(auto i : ord) {
      if(cur>r.at(i)) {
        ok = false;
      }
      cur = max(cur,l.at(i));
    }
    ans += !!ok;
  } while(next_permutation(all(ord)));
  cout << ans << endl;
  
  return 0;
}