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

Example Input: Sentence: 'person wearing all white leaned up against a wall with a yellow sign'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'an image of a street sign with a corner restaurant'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a man holding a large blue frisbee in a yard'. Find frequency of the letter 'o'
Example Output:
1