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

Example Input: Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'
Example Output: 2

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 cat is sitting atop a pile of suitcases'. Find frequency of the letter 's'
Example Output:
5