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

Example Input: Sentence: 'one woman holding a video game controller and standing next to another woman'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a man is taking a picture of a man and woman'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'some people and one male is looking at his cellphone'. Find frequency of the letter 'p'
Example Output:
3