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: 'an commercial airline airplane flies above the clouds'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a man riding a wave on his surfboard'. Find frequency of the letter 'o'
Example Output:
2