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

Example Input: Sentence: 'a bus with two floors belonging to london country'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'two people wearing snowboards sitting on a snowy slope'. Find frequency of the letter 's'
Example Output: 5

Example Input: Sentence: 'an old tv lies broken on a beautiful beach'. Find frequency of the letter 'a'
Example Output:
4