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

Example Input: Sentence: 'a couple is cutting in to a cake at a wedding'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'slices of orange are arranged on a plate'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a police officer who is riding on a horse'. Find frequency of the letter 'p'
Example Output:
1