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 man flying through the air on top of a skateboard'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'red wine being poured into five crystal glasses'. Find frequency of the letter 'd'
Example Output:
2