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

Example Input: Sentence: 'a softball player holding the bat on first base'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a large bathroom with marble floor, counter, and bath'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'pizza on plate cut into fourths with one piece mising'. Find frequency of the letter 't'
Example Output:
5