There are two round tables with n dwarves sitting at each table. Each dwarf has only two friends: his neighbours to the left and to the right. A good wizard wants to seat the dwarves at one round table so that each two neighbours are friends. His magic allows him to make any 2n pairs of dwarves into pairs of friends (the dwarves in a pair may be from the same or from different tables). However, he knows that an evil sorcerer will break n of those new friendships. For which n is the good wizard able to achieve his goal no matter what the evil sorcerer does?Mikhail Svyatlovskiy combinatoricsTournament of Towns