Mystric Balls

Motu wants to learn Cricket from a coach, but firstly coach wants to test his IQ level, so he gave Motu $1$ $Red$ $ball$ and $1$ $Black$ $ball$ , and asked him to buy other $x – 1$ red balls and other $y – 1$ black balls from the market. But he put some conditions on buying balls, that if he has $R$ red and $B$ black balls then he can either buy $B$ red balls or $R$ black balls in one operation. H