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

Example Input: Sentence: 'two people and two children walking in a building'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'three men in military suits are sitting on a bench,'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'there is a bowl that has different color cauliflower'. Find frequency of the letter 'a'
Example Output:
4