Please study the following problems related to combinatorics:

1. http://acm.uva.es/p/v4/485.html
2. http://acm.uva.es/p/v5/530.html
3. http://acm.uva.es/p/v100/10007.html
4. http://acm.uva.es/problemset/v101/10198.html
   
5. http://acm.uva.es/problemset/v102/10213.html
   
6. http://acm.uva.es/problemset/v103/10328.html
   
7. http://acm.uva.es/p/v104/10444.html
8. http://acm.uva.es/p/v104/10446.html
   
9. http://acm.uva.es/p/v106/10696.html