Question 22
The vertices of a convex 1991-gon are enumerated with integers from $1$ to $1991$. Each side and diagonal of the $1991$-gon is coloured either red or blue. Prove that, for an arbitrary renumeration of vertices, one can find integers $k$ and $l$ such that the line connecting vertices with numbers $k$ and $l$ before the renumeration has the same colour as the line between the vertices having these numbers after the renumeration.