Kaprekar conversion

The Indian mathematician D. R. Kaprekar is known for his works in number theory. One of his works is devoted to the so-called Kaprekar transformation. Consider the following operation. Let the number $x$ be given. Let $M$ be the largest number that can be obtained from $x$ by permuting its digits, and $m$ is the smallest number (this number can contain leading zeros). Let's designate as $K(x)$ the difference M - m, supplemented if necessary with leading zeros so that the number of digits in it is equal to the number of digits in $x$.

For example, K(100) = 100 - 001 = 099. K(2414) = 4421 - 1244 = 3177.

Kaprekar proved that if we start with some four-digit number $x$, in which not all the digits are equal, and successively apply this operation to it (calculate K(x), K(K(x)), ...), then sooner or later the number $6174$ will appear. For this number the equality K(6174) = 7641 - 1467 = 6174 is true, therefore, the process will cycle on this number.

Your task is to write a program that calculates $K(x)$ by the number $x$.