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

Example Input: Sentence: 'one woman holding a video game controller and standing next to another woman'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'the grey cat is perched on a red and grey chair'. Find frequency of the letter 'g'
Example Output:
2