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

Example Input: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'a herd of giraffes in tall african grass'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a bathroom that has some open windows in it'. Find frequency of the letter 'o'
Example Output:
5