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

Example Input: Sentence: 'a notebook and pen are left on a park bench'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a white police vehicle parked on a paved street'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'an adult and child flying a kite surrounded by brown vegetation'. Find frequency of the letter 'o'
Example Output:
3