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

Example Input: Sentence: 'group of cars on the road with the sun setting behind it'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a man in a suit makes the first slice into a cake'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'the sign is clearly visible for all of us to see'. Find frequency of the letter 'i'
Example Output:
4