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

Example Input: Sentence: 'a baseball themed birthday cake is on a serving tray'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a white toilet sitting next to a toilet paper roller'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a black cat  some legs in blue jeans and black shoes'. Find frequency of the letter 'l'
Example Output:
4