2802. Find The K-th Lucky Number

The number of lucky numbers with exactly n digits is equal to 2n.

We can obtain how many digits the kth lucky number has.

Imagine we know that kth lucky number has c digits. Then calculate how many numbers with c digits exist before the kth lucky number.

Imagine the number from the previous hint is x. Now look at the binary representation of x and add some leading zero to make its length equal to c.

Replace 0 and 1 with 4 and 7 in the number you've obtained from the previous hint.