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

Example Input: Sentence: 'a half eaten doughnut in a persons hand'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a birthday cake with a number one and three candle'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a sitting cat wearing a collar and stripped tie'. Find frequency of the letter 'g'
Example Output:
2