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

Example Input: Sentence: 'a baseball player dropping his bat and starting to run'. Find frequency of the letter 'o'
Example Output: 2

Example Input: Sentence: 'a cat that is standing next to an open suitcase'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'two boys are sitting upright in bed near a nightstand'. Find frequency of the letter 'y'
Example Output:
1