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

Example Input: Sentence: 'a woman swinging a tennis racquet towards a tennis ball'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a man is taking a picture of a man and woman'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a herd of animals grazing on a lush green field'. Find frequency of the letter 'g'
Example Output:
3