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

Example Input: Sentence: 'a closeup view of a bathroom sink, that is very modern'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a man in a sweatshirt is taking his own picture'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a dog has found a rather large stick for playing fetch'. Find frequency of the letter 't'
Example Output:
3