covariance and correlation

hi, could someone please show me how to answer the following question:

a fair coin is thrown n times, each throw being independant of the ones before. let R=the number of heads, and S=the number of tails. find the covariance of R and S. what is the correlation of R and S?

alhtough my book kind of gives an answer, i dont understand it. i somewhat understand how they got the covariance of -0.25 using Xi A as the no of heads and Yi as the no of tails in the ith throw, but i dont know how to calculate the correlation from there. i know you have to use the rule:

cov(X+W, Y+V) = cov (X,Y) + cov(W,V)

but dont know how to proceed from there...

thanks