Problem

Source: Bulgarian Autumn Tournament 2024, 10.4

Tags: directed graph, game, combinatorics



Let $G$ be a complete directed graph with $2024$ vertices and let $k \leq 10^5$ be a positive integer. Angel and Boris play the following game: Angel colors $k$ of the edges in red and puts a pawn in one of the vertices. After that in each move, first Angel moves the pawn to a neighboring vertex and then Boris has to flip one of the non-colored edges. Boris wins if at some point Angel can't make a move. Find, depending on $G$ and $k$, whether or not Boris has a winning strategy.