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

Example Input: Sentence: 'the small black and white dog holds onto a frisbee'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a person riding a surfboard in the middle of a wave'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a woman in a defensive stance holding a tennis racquet'. Find frequency of the letter 'e'
Example Output:
6