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

#ifdef LOCAL
#define debug(x) cerr << #x << " = " << (x) << "\n"
#else
#define debug(x) void(0)
#endif

typedef long long ll;
typedef long double ld;
typedef pair<int, int> Pii;
typedef pair<ll, ll> Pll;

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
	for (int i = 0; i < (int)v.size(); i++)
		os << v[i] << (i + 1 == (int)v.size() ? "" : " ");
	return os;
}

template <typename T>
istream &operator>>(istream &is, vector<T> &v)
{
	for (int i = 0; i < (int)v.size(); i++)
		is >> v[i];
	return is;
}

////////////////////////////////////////////////////////////

int main()
{
	int N;
	cin >> N;
	vector<int> L(N), R(N);
	for (int i = 0; i < N; i++)
		cin >> L[i] >> R[i];
	vector<int> idx(N);
	iota(idx.begin(), idx.end(), 0);
	int ans = 0;
	do
	{
		bool ok = true;
		int cur = 0;
		for (int i = 0; i < N; i++)
		{
			if (cur > R[idx[i]])
			{
				ok = false;
				break;
			}
			cur = max(cur, L[idx[i]]);
		}
		ans += ok;
	} while (next_permutation(idx.begin(), idx.end()));
	cout << ans << "\n";
	return 0;
}