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

Example Input: Sentence: 'a view from a bathroom shows an open door'. Find frequency of the letter 'o'
Example Output: 7

Example Input: Sentence: 'the skateboarder is jumping over a huge statue of a shoe'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'a pair of stone buildings and a clock'. Find frequency of the letter 'a'
Example Output:
4