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

Example Input: Sentence: 'three young children playing with a ball in front of a huge bridge'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'an old glass vase containing flowers on a glass table'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a white toilet bowl next to a glass shower'. Find frequency of the letter 'e'
Example Output:
4