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

Example Input: Sentence: 'two boys are doing skateboard jumps in the street'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a snow filled mountain with a guy with ski's'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a young girl holds her colorful butterfly kite'. Find frequency of the letter 'l'
Example Output:
5