#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    int N;
    std::cin >> N;
    std::vector<int> L(N), R(N);
    for (int i = 0; i < N; i++) std::cin >> L[i] >> R[i];
    std::vector<int> order(N);
    for (int i = 0; i < N; i++) order[i] = i;
    int result = 0;
    do {
        bool flag = true;
        int diff = L[order[0]];
        for (int i = 1; i < N; i++) {
            if (diff > R[order[i]]) {
                flag = false;
                break;
            }
            if (diff < L[order[i]]) diff = L[order[i]];
        }
        result += flag;
    } while (std::next_permutation(order.begin(), order.end()));
    std::cout << result << std::endl;
}