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

Example Input: Sentence: 'person wearing all white leaned up against a wall with a yellow sign'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a plane with stairs next to it sitting in a large lot'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a barefoot person sitting next to a desk and a laptop'. Find frequency of the letter 'd'
Example Output:
2