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

Example Input: Sentence: 'people waiting on a train platform in a wooded area'. Find frequency of the letter 'p'
Example Output: 3

Example Input: Sentence: 'a person riding a skate board on a city street'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a large bowl of food is sitting on the table'. Find frequency of the letter 'o'
Example Output:
5