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

Example Input: Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'the skateboarder has on red and black sneakers and is riding a skateboard'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'the person in grey jacket and red pants is standing next to the suitcase in the forest'. Find frequency of the letter 't'
Example Output:
10