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

Example Input: Sentence: 'an image of a street sign with a corner restaurant'. Find frequency of the letter 'u'
Example Output: 1

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 woman hitting a ball with the tennis racket at the tennis court'. Find frequency of the letter 's'
Example Output:
2