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

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: 'supplies and tools are arranged on the floor with a hard hat and bag'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a grass field with birds resting on it'. Find frequency of the letter 'i'
Example Output:
5