Please solve the programming task below using a self-contained code snippet in a markdown code block.

You are given a binary string s and a positive integer k.
Return the length of the longest subsequence of s that makes up a binary number less than or equal to k.
Note:

The subsequence can contain leading zeroes.
The empty string is considered to be equal to 0.
A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.

 
Please complete the following python code precisely:
```python
class Solution:
    def longestSubsequence(self, s: str, k: int) -> int:
```