Changeling

A number $P$ is called the reverse of a number $N$ if the reversed decimal representation of one matches the decimal representation of the other. For example, the reverse of $3489$ is $9843$, and the reverse of $2009100$ is $0019002$ or, excluding leading zeros, simply $19002$. You are given a positive integer $N$. Find the largest integer $M$ such that $M$ added to its reverse equals the given number $N$.