Challenge 18

Polka Dots on the Straight

Difficulty level:

In a straight line there are 1999 balls. Some are green and the others blue (could be all green or all blue). Under each ball we write the number equal to the sum of the number of green balls on her right plus the amount of blue balls on her left. If in the sequence of numbers thus obtained there are exactly three numbers that appear an odd number of times, what can these three numbers be?