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

Example Input: Sentence: 'two people wearing snowboards sitting on a snowy slope'. Find frequency of the letter 's'
Example Output: 5

Example Input: Sentence: 'a woman in yellow dress standing next to a horse'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'an owner plays tug-of-war with their golden retriever'. Find frequency of the letter 'o'
Example Output:
3