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

Example Input: Sentence: 'a picture of two computers on a table'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a bowl of granola, dried fruit, and bananas'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a white toilet sitting next to a bathroom sink'. Find frequency of the letter 'b'
Example Output:
1