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 silver train crossing the road on tracks'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'plates of chicken and vegetable, a plate with bread and a bowl of soup'. Find frequency of the letter 'a'
Example Output:
8