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

Example Input: Sentence: 'two laptop computers sitting on top of a white table'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a microwave oven with a yellow note on the door'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'man looking up from magazine reading sitting at a cafe table'. Find frequency of the letter 'i'
Example Output:
5