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

Example Input: Sentence: 'a traffic signal by a wire filled with some birds'. Find frequency of the letter 'e'
Example Output: 3

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 large brown dog sitting in the back seat of a car'. Find frequency of the letter 'a'
Example Output:
6