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

Example Input: Sentence: 'a blue fire hydrant painted on the bottom of it'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a man with a large grey and black beard and glasses and a tie'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a large flowers sign hanging from the side of a tall building'. Find frequency of the letter 'n'
Example Output:
4