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

Example Input: Sentence: 'the sun goes down over a bike tied to a bench at the beach'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'three birds are sitting on a tree in a forest'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'several cows with tagged ears standing in a grassy field'. Find frequency of the letter 's'
Example Output:
6