For a positive integer $n$ and nonzero digits $a,b,c,d$, let $A_n$ be the $n$-digit integer each of whose digits is equal to $a$, let $B_n$ be the $n$-digit integer each of whose digits is equal to $b$, let $C_n$ be the $2n$-digit integer each of whose digits is equal to $c$, and let $D_n$ be the $n$-digit integer each of whose digits is equal to $d$. What is the greatest possible value of $a+b+c+d$ for which there are at least two values of $n$ such that $C_n-A_n=B_n{D}_n$?