Keep it simple ….

You are a vendor having some balls:-

1) 1994 RED balls
2) 1995 GREEN balls
3) 1996 BLUE balls

now if you tak two balls of different colors , then you can convert them to 1 ball of the third color

eg. one RED and one GREEN ball combine to form one BLUE ball

now you have to convert all the balls into one single color.

How will you do this ?