In this task, you need to count the number of times the given letter appears in the given sentence.

Example Input: Sentence: 'a bird outside standing on a table looking up'. Find frequency of the letter 'b'
Example Output: 2

Example Input: Sentence: 'a bunch of green bananas is hanging on a tree'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'freight train traveling beside a road through a countryside'. Find frequency of the letter 'n'
Example Output:
3