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

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

Example Input: Sentence: 'this is an image of a woman playing tennis'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'an decorated white elephant statue with a bull's eye painted on it'. Find frequency of the letter 'a'
Example Output:
6