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: 'the skateboarder is jumping over a huge statue of a shoe'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'a piece of luggage and it's contents sitting next to a fire place'. Find frequency of the letter 'n'
Example Output:
5