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

Example Input: Sentence: 'a stuffed animal sitting on an empty bed'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a man with headphones on is hunched over a sink while being recorded'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'a tall coffe cup sitting on a table next to an open laptop'. Find frequency of the letter 'p'
Example Output:
4