Odd goings on |

In a party with *n* people, it is known that for every nonempty subset *S* of people, there is at least one person, inside or outside *S*, such that this person has an odd number of friends in *S*. Prove that *n* is even.

Odd goings on |

*n* people, it is known that for every nonempty subset *S* of people, there is at least one person, inside or outside *S*, such that this person has an odd number of friends in *S*. Prove that *n* is even.