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: 'a lady mixes some things in a blender as people look on'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'brown and white monkeys in a tree eat some bananas'. Find frequency of the letter 'n'
Example Output:
6