Attendees of a faculty party shake hands to greet each other, and each professor remembers how many

Question:

Attendees of a faculty party shake hands to greet each other, and each professor remembers how many times he or she shook hands. At the end of the party, the department head adds up the number of times that each professor shook hands. Show that the result is even by proving the handshaking lemma: if G = (V, E) is an undirected graph, then