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

Example Input: Sentence: 'a couple of items that are on the wall'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'young boy flying kite in large field at outdoor event'. Find frequency of the letter 'y'
Example Output: 3

Example Input: Sentence: 'the two dogs are playing in the pen together'. Find frequency of the letter 'o'
Example Output:
3