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 woman standing in front of a bathroom sink in front of a mirror'. Find frequency of the letter 'o'
Example Output: 8

Example Input: Sentence: 'there is a short train moving down the tracks'. Find frequency of the letter 'a'
Example Output:
3