Graph Theory Homework Help

hey everybody..i am new to this site and i was wondering if anyone might be able to help me with a Graph Theory problem that i just can't seem to figure out. The question asks me to "Find the number of different perfect matchings in a complete graph with 2n vertices." After working with some classmates we have come to a few ideas, but none of us are sure and this homework is worth a big chunk of our grade. We even went to see our teacher but he wasn't much help. HELP.