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

Example Input: Sentence: 'a white paper plate with a slice of pizza on it'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a horse drawn carriage sitting in the middle of a street'. Find frequency of the letter 'd'
Example Output:
3