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

Example Input: Sentence: 'a black horse with a white dot on its forehead'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'there are two men sitting on a couch playing video games'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a back pack on a chair  and a person sitting next to it'. Find frequency of the letter 'i'
Example Output:
4