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

Example Input: Sentence: 'a person on a sidewalk jumping with an umbrella'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a bird on a tree is flapping its wings'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a stop sign in the desert near an empty road'. Find frequency of the letter 'e'
Example Output:
5