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

Example Input: Sentence: 'flower in a vase full of water sitting on a stand '. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a girl sits on a surfboard and smiles'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a bathroom shot inside of a bathroom'. Find frequency of the letter 'h'
Example Output:
3