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

Example Input: Sentence: 'a couple of items that are on the wall'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'soccer players on opposite teams fight for control over a ball'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a man nurse with gloves on is helping a woman'. Find frequency of the letter 'w'
Example Output:
2