A number is called lucky if it comprises only the digits $4$ and $7$. Your task is to return the k-th lucky number as a string for a given integer k. 

Constraints:

• $1 <=$ k $<= 10^9$

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding on how to solve this problem.

Implement your solution in the following coding playground.