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

int main() {
    int N;
    cin >> N;
    int idx[N];
    int L[N], R[N];
    for (int i = 0; i < N; i++) {
        idx[i] = i;
        cin >> L[i] >> R[i];
    }

    int ans = 0;
    do {
        bool ok = true;
        int l = L[idx[0]];
        for (int i = 1; i < N; i++) {
            int nl = L[idx[i]], nr = R[idx[i]];
            if (nr < l) {
                ok = false;
                break;
            }
            l = max(l, nl);
        }
        ans += ok ? 1 : 0;
    } while (next_permutation(idx, idx + N));

    cout << ans << '\n';
}