IMO Shortlist 2018 problem C5
Let be a positive integer. The organising commitee of a tennis tournament is to schedule the matches for players so that every two players play once, each day exactly one match is played, and each player arrives to the tournament site the day of his first match, and departs the day of his last match. For every day a player is present on the tournament, the committee has to pay coin to the hotel. The organisers want to design the schedule so as to minimise the total cost of all players' stays. Determine this minimum cost.