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

Example Input: Sentence: 'a variety of a different type of vegetables'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'an odd looking black duck with a colorful beak'. Find frequency of the letter 'l'
Example Output:
4