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

Example Input: Sentence: 'a wedding cake with pink roses on a silver plate'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'an old green wooden clock stands in an old hallway'. Find frequency of the letter 'i'
Example Output:
1