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

Example Input: Sentence: 'a little child looking up to a camera holding a cell phone'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a bus is driving on the street behind some cars'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'a women who is taking a drink of a beer'. Find frequency of the letter 'a'
Example Output:
4