The vertices of a convex 1991-gon are enumerated with integers from 1 to 1991. Each side and diagonal of the 1991-gon is colored either red or blue. Prove that, for an arbitrary renumeration of vertices, one can find integers k and l such that the segment connecting the vertices numbered k and l before the renumeration has the same color as the segment connecting the vertices numbered k and l after the renumeration.