Školjka

%V0 $n \geq 4$ players participated in a tennis tournament. Any two players have played exactly one game, and there was no tie game. We call a company of four players $bad$ if one player was defeated by the other three players, and each of these three players won a game and lost another game among themselves. Suppose that there is no bad company in this tournament. Let $w_i$ and $l_i$ be respectively the number of wins and losses of the $i$-th player. Prove that $$\sum^n_{i=1} \left(w_i - l_i\right)^3 \geq 0.$$ Proposed by Sung Yun Kim, South Korea