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

Example Input: Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'one woman is arranging the other woman's hair'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a large body of water filled with boats next to a tall building'. Find frequency of the letter 'y'
Example Output:
1