#import<bits/stdc++.h>
using namespace std;int n,a,m,i=1,l[8],r[8],t[8];int main(){for(cin>>n;i<n;i++)cin>>l[i]>>r[i],t[i]=i;do{m=0;for(int i:t)m=m>r[i]?1e7:max(m,l[i]);m-1e7&&a++;}while(next_permutation(t,t+n));cout<<a;}