suppose no element fit this except .
you may consider all the pairs and "join" them by a line.
Then for all , there is exactly one line connected to it.
if is only possible for ,then all the other lines will occupy two elements, and the line occupys one element. Thus must be an odd number!!!
Suppose this is not true. Since the inverse of 1 is itself, that leaves an odd number of elements such that each is NOT its own inverse. Pair each element with its inverse. What happens? (Of course, you need the fact that inverses are unique- if a*b= 1 and a*c= 1, then b= c.)