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

Example Input: Sentence: 'a batter, catcher and umpire waiting for the pitch and a major league baseball game'. Find frequency of the letter 'c'
Example Output: 3

Example Input: Sentence: 'an orange cat sitting on the ground next to a  purse'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a kitchen with two pots on the stove with lids'. Find frequency of the letter 'w'
Example Output:
3