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

Example Input: Sentence: 'a couple of boys in a field playing baseball'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a person is riding in a cart pulled by a horse'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'four jet fighters passing by in the air in formation'. Find frequency of the letter 'n'
Example Output:
4