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

Example Input: Sentence: 'there is a stop sign that has a reflection in a puddle'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a couple of buildings that has a bus out front'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a man walking in front of a cinder block building near a stop sign'. Find frequency of the letter 'i'
Example Output:
6