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

Example Input: Sentence: 'a picture of a machine built next to a street'. Find frequency of the letter 'x'
Example Output: 1

Example Input: Sentence: 'two little girls playing with a bat and a tennis ball'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'a man is swinging a baseball bat on the diamond'. Find frequency of the letter 'n'
Example Output:
5