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

Input: Consider Input: Sentence: 'a red train traveling down train tracks near a forest'. Find frequency of the letter 'f'

Output: 1

Input: Consider Input: Sentence: 'a pizza is in a box on a table outside'. Find frequency of the letter 'u'

Output: 1

Input: Consider Input: Sentence: 'an avocado, an apple, a banana, a pear, and an orange'. Find frequency of the letter 'a'
Output: 14