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

Example Input: Sentence: 'two bigger elephants standing with a small elephant'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a glass bowl of raw broccoli, a ceramic bowl of raw green chili peppers, and steel bowl of raw daikon'. Find frequency of the letter 'a'
Example Output: 9

Example Input: Sentence: 'a dog laying underneath a computer desk with a laptop computer and monitor on top of it'. Find frequency of the letter 'o'
Example Output:
9