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

Example Input: Sentence: 'the bathroom is in need of remodel'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a jet that is flying in the sky with a banner behind it'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'small boys playing baseball on a sandy field'. Find frequency of the letter 'i'
Example Output:
2