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

Example Input: Sentence: 'a person that is looking out his window'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'the wooden kitchen has a small child's plastic table in it'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a pair of giraffe crossing their necks and looking at cars'. Find frequency of the letter 'c'
Example Output:
3