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

Example Input: Sentence: 'a pizza is in a box on a table outside'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'an old glass vase containing flowers on a glass table'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a grey and white pigeon sitting on a branch'. Find frequency of the letter 'a'
Example Output:
4