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

Example Input: Sentence: 'the child is waving a small flag as she stands near two women'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a man in his car eating a donut'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a tan and green motorcycle grouped together with other motorcycles by a bike rack'. Find frequency of the letter 'e'
Example Output:
9