The function f(x) is a simple substitution cipher that replaces each letter in the input string with a letter a fixed number of positions down the alphabet. For example, if the input string is "hello", the function will replace the letter "h" with the letter "o" (which is 3 positions down the alphabet), replace the letter "e" with the letter "t" (which is 4 positions down the alphabet), and so on.

Here is some example code that approximates the behavior of the function:

