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

Example Input: Sentence: 'a little boy sitting on a bed in a bedroom'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a bird outside standing on a table looking up'. Find frequency of the letter 'b'
Example Output: 2

Example Input: Sentence: 'a person on a motor bike on a road'. Find frequency of the letter 'i'
Example Output:
1