2005 points On a horizontal line, black or white
Source: Estonia IMO TST 2005 p5
March 28, 2020
Coloringcombinatorics
Problem Statement
On a horizontal line, points are marked, each of which is either white or black. For every point, one finds the sum of the number of white points on the right of it and the number of black points on the left of it. Among the sums, exactly one number occurs an odd number of times. Find all possible values of this number.