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: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'someone in a red hat standing on a snowy hill'. Find frequency of the letter 'o'
Example Output:
4