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

Example Input: Sentence: 'a closet with a fan, luggage and various boxes'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a person is riding in a cart pulled by a horse'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'political statements pasted to the back of a street sign'. Find frequency of the letter 'o'
Example Output:
3