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

Example Input: Sentence: 'man is snow gear and skis skiing in the snow'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'apples, pears, and oranges are piled up for sale'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'men are playing baseball in an open field'. Find frequency of the letter 'e'
Example Output:
5