#include <bits/stdc++.h>

static inline constexpr bool check(const std::vector<std::pair<uint_fast32_t, uint_fast32_t>>& range_of, const std::vector<uint_fast32_t>& p) noexcept
{
	uint_fast32_t prev_diff = 0;
	for (const auto& cur : p)
	{
		const auto& [l, r] = range_of[cur];
		if (prev_diff > r) return false;
		else prev_diff = std::max(prev_diff, l);
	}

	return true;
}

static inline constexpr uint_fast32_t solve(const uint_fast32_t N, const std::vector<std::pair<uint_fast32_t, uint_fast32_t>>& range_of) noexcept
{
	uint_fast32_t ans = 0;
	std::vector<uint_fast32_t> p(N);
	std::iota(p.begin(), p.end(), 0);
	do
	{
		if (check(range_of, p))
			++ans;
	}
	while (std::next_permutation(p.begin(), p.end()));

	return ans;
}

int main()
{
	std::cin.tie(nullptr);
	std::ios::sync_with_stdio(false);
	
	uint_fast32_t N;
	std::cin >> N;
	std::vector<std::pair<uint_fast32_t, uint_fast32_t>> range_of(N);
	for (auto& [l, r] : range_of)
		std::cin >> l >> r;

	std::cout << solve(N, range_of) << '\n';
	return 0;
}