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

Example Input: Sentence: 'a person working at an airport, outside of an airplane'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'two persons standing outside a moving sub train'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'urinals lining a wall in an industrial-looking bathroom'. Find frequency of the letter 'u'
Example Output:
2