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

int main(){
	int n,r(0); cin >> n; vector<pair<int,int>> A(n); for (auto& [a,b]:A) cin >> a >> b ;
	vector<int> B(n); iota(B.begin(),B.end(),0);
	do {
		int x = 0,rr = 1;
		for (int i:B){
			x = max(x,A[i].first);
			if (x>A[i].second) rr = 0;
		}
		r += rr;
	} while(next_permutation(B.begin(),B.end()));
	cout << r << endl;
}