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

Example Input: Sentence: 'a photo of a small boat on a shore'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a herd of giraffe walking across a field'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a bathroom with a white toilet sitting next to a sink under a mirror'. Find frequency of the letter 'o'
Example Output:
5