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

Example Input: Sentence: 'three elephants walk up a dirt trail from the water'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a little boy standing near a couch and a table'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'a black and white photograph of a tied up cow'. Find frequency of the letter 'p'
Example Output:
3