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

Example Input: Sentence: 'a tennis player that is running to get the tennis ball'. Find frequency of the letter 't'
Example Output: 7

Example Input: Sentence: 'a red stop sign with street signs above it'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a white plate topped with a cut in half sandwich an a pile of fries'. Find frequency of the letter 'e'
Example Output:
5