#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define all(obj) (obj).begin(), (obj).end()
using ll = long long;
using namespace std;

int main() {
    int N;cin>>N;
    vector<int> L(N),R(N);
    rep(i,N) cin >> L[i]>>R[i];
    int ans = 0;
    vector<int> p(N);
    rep(i,N) p[i] = i;
    do{
        vector l = L;
        vector r = R;
        bool flag = true;
        for(int i = 0; i < N-1; i++){
            int c = p[i];
            int n = p[i+1];

            if(l[c] > r[n]){
                flag = false;
                break;
            }else if(l[c] > l[n]){
                l[n] = l[c];
            }else{
                ;
            }
        }
        if( flag ){
            ans++;
        }

    }while(next_permutation(all(p)));
    cout << ans << endl;
}