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

Example Input: Sentence: 'a herd of giraffes in tall african grass'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a man flying through the air on a bicycle'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'many people behind a stand selling bananas'. Find frequency of the letter 'n'
Example Output:
6