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

Example Input: Sentence: 'a surfer wipes out on a huge wave'. Find frequency of the letter 'a'
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: 'many different types of toy trucks next to each other'. Find frequency of the letter 't'
Example Output:
7