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

Example Input: Sentence: 'a plastic container next to a toilet on a wooden box'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'the little girl is flying her kite in the field'. Find frequency of the letter 'i'
Example Output: 7

Example Input: Sentence: 'an office table holding a monitor and keyboard in a room'. Find frequency of the letter 'a'
Example Output:
6