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

Example Input: Sentence: 'a family out snow skiing on a blustery day'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'red chair next to side table with lamp in residential home'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a jumbo jet plane flying below a view of the moon'. Find frequency of the letter 'i'
Example Output:
2