#include #include #include using namespace std; using ll = long long; int main(){ int n; cin >> n; vectorl(n),r(n); vectorp(n); for(int i=0;i> l[i] >> r[i]; p[i]=i; } int ans=0; do{ int x=0; bool ok=true; for(int i=0;ir[p[i]]){ ok=false; break; } x=max(x,l[p[i]]); } ans+=ok; }while(next_permutation(p.begin(),p.end())); cout << ans << endl; return 0; }