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

Example Input: Sentence: 'many different species of sheep in display at a museum'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'a group of giraffes walk together by a hill'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a boy containing two bananas and three oranges'. Find frequency of the letter 'o'
Example Output:
4