Two coloured points are marked on a line, with the blue one at the left and the red one at the right. You may add to the line two neighbouring points of the same color (both red or both blue) or delete two such points (neighbouring means that there is no coloured point between these two). Prove that after several such transformation you cannot again get only two points on the line in which the red one is at the left and the blue one is at the right.(A Belov)
combinatoricscombinatorial geometryColoring