This is a good place to find good challenges. I have used a few of these in my perlgolf challenges.

http://acm.uva.es/problemset/

Terje k

I like this one.

http://acm.uva.es/p/v106/10608.html

Just simplify it a bit. only one set of data and just the lines with friends. no need for number of ppl / lines in input.

input :
1 2
3 1
3 4
5 4
3 5
4 6
5 2
2 1
7 10
1 2
9 10
8 9

gives output :
6

Terje K