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

Example Input: Sentence: 'a railroad train pulling up to the train platform'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a bathroom that has it's lights turned on'. Find frequency of the letter 'h'
Example Output: 4

Example Input: Sentence: 'a very clean, modern and minimalistic style bedroom'. Find frequency of the letter 'e'
Example Output:
5