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

Example Input: Sentence: 'a child sitting a table with a pizza in front of him'. Find frequency of the letter 'h'
Example Output: 3

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 small girl enthusiastically plays with a wii controller'. Find frequency of the letter 'i'
Example Output:
6