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

Example Input: Sentence: 'a tennis player after hitting a shot back to the other side of the court'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'two young men dressed up in suit and ties'. Find frequency of the letter 's'
Example Output:
4