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: 'the meal is served and ready to be eaten'. Find frequency of the letter 'e'
Example Output: 8

Example Input: Sentence: 'a skier is going downhill on a wooded slope'. Find frequency of the letter 'd'
Example Output:
3