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

Example Input: Sentence: 'a man swinging a bat while playing baseball'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'two men sitting quietly on a park bench'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'a man in striped shirt swinging a baseball bat'. Find frequency of the letter 'e'
Example Output:
2

