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

Example Input: Sentence: 'a man and a woman are walking down the street with an umbrella over their heads'. Find frequency of the letter 'w'
Example Output: 4

Example Input: Sentence: 'a person trying to flip a skateboard in a garage'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a dog looking ahead with a stoic look in a car seat'. Find frequency of the letter 'r'
Example Output:
1