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

Example Input: Sentence: 'a half eaten doughnut in a persons hand'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a bathroom that has it's lights turned on'. Find frequency of the letter 'h'
Example Output: 4

Example Input: Sentence: 'two cows grazing in a field surrounded by a fence'. Find frequency of the letter 'a'
Example Output:
3