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

Example Input: Sentence: 'a bathroom that has it's lights turned on'. Find frequency of the letter 'h'
Example Output: 4

Example Input: Sentence: 'a baseball player dropping his bat and starting to run'. Find frequency of the letter 'o'
Example Output: 2

Example Input: Sentence: 'a dog on a cobbled walkway holding two bottles in its mouth'. Find frequency of the letter 'h'
Example Output:
2