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

Example Input: Sentence: 'a man playing baseball on a television set'. Find frequency of the letter 'l'
Example Output: 4

Example Input: Sentence: 'bananas are being dried in a tropical setting'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a woman places her order at a food court restaurant'. Find frequency of the letter 'n'
Example Output:
2