#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n; cin >> n;
	vector<int>v(n);
	rep(i, n)v[i] = i;
	vector<int>l(n), r(n);
	rep(i, n)cin >> l[i] >> r[i];
	int ans = 0;
	// 順列
	do {
		int diff = -INF;
		bool chk = true;
		rep(i, n) {
			if (r[v[i]] <= diff) {
				chk = false;
				break;
			}
			diff = max(diff, l[v[i]]);
		}
		if (chk)ans++;
	} while (next_permutation(v.begin(), v.end()));
	cout << ans << endl;
	return 0;
}