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