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

Example Input: Sentence: 'a toilet of some sort near a sink of some sort'. Find frequency of the letter 'o'
Example Output: 7

Example Input: Sentence: 'people seated on a table looking very happy'. Find frequency of the letter 'g'
Example Output: 1

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