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

Example Input: Sentence: 'two zebra stand near the wooden posts on a building'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a man riding a white surfboard on a large wave'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a man wearing glasses sits down to a meal with lots of bottles of wine'. Find frequency of the letter 'n'
Example Output:
4