25.2. (USA, 82). In a society consisting of 1982 people, among any four people, at least one can be chosen who is acquainted with the other three. What is the minimum possible number of people who are acquainted with everyone?
25.2. If there are no strangers, then the number of people who know everyone is 1982. Let $A$ and $B$ not know each other. Then all other people know each other (if $C$ does not know $D$, then in the group $A, B, C, D$ no one knows the other three). If $A$ and $B$ know all the others, then 1980 people know everyone. If $A$ does not know $C$ as well, where $C \neq B$, then $A$, $B$, and $C$ are not familiar with all the other 1979 people (since in any group $A, B, C, D$ only $D$ can be familiar with the other three), who are also familiar with each other. Thus, the minimum number of people who know everyone is 1979.