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

Example Input: Sentence: 'a woman sitting in front of a giant slice of cheese pizza'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'cars parked outside if a retail store'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a man is riding a skateboard on a concrete surface'. Find frequency of the letter 'e'
Example Output:
4