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

Example Input: Sentence: 'oranges are sitting in a bowl on a table'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a herd of zebra standing on top of a grass covered field'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a man looks at a huge pile of green bananas'. Find frequency of the letter 'e'
Example Output:
4