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

Example Input: Sentence: 'a burning wood stove has a tea pot sitting on top of it and is full of wood'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a man sitting on a motorcycle near two other men'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a clear shot of a bathroom in a hotel room'. Find frequency of the letter 'o'
Example Output:
7