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

Example Input: Sentence: 'man in a sleeping bag on the floor using a laptop computer'. Find frequency of the letter 'i'
Example Output: 3

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: 'a child standing in front of a television with a remote'. Find frequency of the letter 'i'
Example Output:
6