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

Example Input: Sentence: 'a woman holds some baked goods at the convention'. Find frequency of the letter 'd'
Example Output: 3

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: 'two identical photos of a lady about to hit a ball with a tennis racket'. Find frequency of the letter 't'
Example Output:
9