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

Example Input: Sentence: 'this marketplace has a lot of fresh fruit and vegetables'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'two woman sitting on the couch in  a living room'. Find frequency of the letter 'o'
Example Output:
6