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

Example Input: Sentence: 'a person is riding in a cart pulled by a horse'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'there is no image here to provide a caption for'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a woman is holding a bunch of bananas'. Find frequency of the letter 'i'
Example Output:
2