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

Example Input: Sentence: 'salad and vegetables with me on top of plate'. Find frequency of the letter 'o'
Example Output: 3

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

Example Input: Sentence: 'a man standing at the end of a tennis court while looking down'. Find frequency of the letter 't'
Example Output:
5