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

Example Input: Sentence: 'a man with a large grey and black beard and glasses and a tie'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'two men sitting quietly on a park bench'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'pedestrian sign and bus sign in urban area'. Find frequency of the letter 'g'
Example Output:
2