Palindromic number

You are given a string $S$ consisting of $N$ digits from $0$ to $9$. You want to obtain a palindrome from $S$ using some operations. There are $M$ different types of operations you can make on $S$. Each operation is described by three integers $X$, $Y$ and $W$. In this operation, if there is a digit equal to $X$ in $S$, you can replace $X$ with $Y$ using $W$ coins. For example, if $X = 4, Y= 5,