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

Example Input: Sentence: 'a man that is on his cell phone'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a giraffe sticking it's tongue out near a window'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a woman dressed in brown expressing a curious look'. Find frequency of the letter 's'
Example Output:
5