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 small white dog looking out a car window'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a group of chefs in a kitchen preparing food'. Find frequency of the letter 'i'
Example Output:
3