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 people stand inside of a big room'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'two men in black wet suits beside a white surfboard'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a young couple all dressed up for a formal event'. Find frequency of the letter 'e'
Example Output:
5