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

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N, ans = 0;
    cin >> N;
    vector<int> L(N), R(N), idx(N);
    for(int i=0; i<N; i++) cin >> L[i] >> R[i];
    iota(idx.begin(), idx.end(), 0);
    do {
        int x = 0;
        bool ok = true;
        for(int i=0; i<N; i++) {
            if(x > R[idx[i]]) ok = false;
            x = max(x, L[idx[i]]);
        }
        ans += ok;
    } while(next_permutation(idx.begin(), idx.end()));
    cout << ans << endl;
}