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

Example Input: Sentence: 'a couple of items that are on the wall'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a person with some scissors over a bowl'. Find frequency of the letter 's'
Example Output: 6

Example Input: Sentence: 'a truck with odd patterns and designs on it'. Find frequency of the letter 's'
Example Output:
3