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

Example Input: Sentence: 'the interior of a living and dining room'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a couple of men standing inside of a liquor store'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'a couple of giraffes walk through some grass land'. Find frequency of the letter 'f'
Example Output:
3