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

Example Input: Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a white toilet sitting next to a toilet paper roller'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a photo of a ferry on the water with mountains in the background'. Find frequency of the letter 'f'
Example Output:
2