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

Example Input: Sentence: 'an orange cat sitting on the ground next to a  purse'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a gray elephant is in a grassy field'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a suitcase sits next to the pole of a street sign'. Find frequency of the letter 'e'
Example Output:
6