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

Example Input: Sentence: 'a man on stiltz is standing under a pink umbrella'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a toilet of some sort near a sink of some sort'. Find frequency of the letter 'o'
Example Output: 7

Example Input: Sentence: 'a bathroom with a sink, counter top, and mirror is shown'. Find frequency of the letter 'i'
Example Output:
4