#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    vector<pair<int,int>> a(n);
    for(auto &&[l, r] : a) cin >> l >> r;
    vector<int> ord(n);
    iota(ord.begin(), ord.end(), 0);
    int ans = 0;
    do{
        int v = 0;
        for(auto idx : ord){
            auto [l, r] = a[idx];
            if(v > r){
                v = -1;
                break;
            }
            v = max(v, l);
        }
        ans += (v != -1);
    }while(next_permutation(ord.begin(), ord.end()));
    cout << ans << '\n';
}