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

Example Input: Sentence: 'some cubbies with towels in a blue bathroom'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'two little girls playing with a bat and a tennis ball'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'a black and white picture of two men playing a tennis match with the umpire and spectators look on'. Find frequency of the letter 'e'
Example Output:
7