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

Example Input: Sentence: 'a bowl of vegetables in a soup like substance next to a spoon'. Find frequency of the letter 'a'
Example Output: 5

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 clock mounted to a ceiling next to a rail platform'. Find frequency of the letter 't'
Example Output:
5