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

Example Input: Sentence: 'a closeup view of a bathroom sink, that is very modern'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'many white birds flying above a river that flows in front of a forest'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a girl riding a skateboard while crouching and a man on one in the background'. Find frequency of the letter 'i'
Example Output:
6