Chunk’s chunky Party

This is the holiday season, and the chunk is inviting the best coders to his annual winter party. He invites $N$ people, and they all enter one after the other. The chunk has arranged a ludo competition in his party, and needs to divide all his friends into pairs to play the game. Note : if any invitee is left unpaired, that person will play against the chunk, otherwise the chunk will not partic