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

Example Input: Sentence: 'an image of a cautionary construction sign'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a closeup view of a bathroom sink, that is very modern'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'food sitting in bowls on table with checked tablecloth'. Find frequency of the letter 't'
Example Output:
6