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

Example Input: Sentence: 'some giraffes moving across a big grass field'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'some cubbies with towels in a blue bathroom'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a white clock tower stands in front of some palm trees'. Find frequency of the letter 's'
Example Output:
4