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

Example Input: Sentence: 'man riding a skate board down the little board way'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a small boy in a high chair eating a cupcake'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'this simple train and train terminal are an example of life in the past'. Find frequency of the letter 'a'
Example Output:
8