#include <bits/stdc++.h>

using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int> L(N), R(N);
    for (int i = 0; i < N; i++) {
        cin >> L[i] >> R[i];
    }
    int ans = 0;
    vector<int> A(N);
    iota(A.begin(), A.end(), 0);
    do {
        int tmp = L[A[0]];
        bool ok = true;
        for (int i = 1; i < N; i++) {
            ok &= tmp <= R[A[i]];
            tmp = max(tmp, L[A[i]]);
        }
        ans += ok;
    } while (next_permutation(A.begin(), A.end()));
    cout << ans << endl;
    return 0;
}