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

Example Input: Sentence: 'a man is taking a picture of a man and woman'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
Example Output: 2

Example Input: Sentence: 'an oven that is opened up outside of a house'. Find frequency of the letter 'n'
Example Output:
3