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

Example Input: Sentence: 'many white birds flying above a river that flows in front of a forest'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'bathroom scene with a black cat going behind the door'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'an apple and banana on top of the table'. Find frequency of the letter 'e'
Example Output:
3