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

Example Input: Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'
Example Output: 4

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: 'a woman standing on a street corner next to a traffic light'. Find frequency of the letter 't'
Example Output:
7