IMO fever
Source: 2016 Japan TST P6
January 26, 2023
combinatorics
Problem Statement
There are people. Initially, only one of these people is infected with IMO fever, but we don't know who it is. When two people shake hands, if one of them is infected with IMO fever, the other person get infected with it. Determine the minimum number of handshakes needed to infect all people, regardless of who the first infected person is.