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

Example Input: Sentence: 'a bus is driving on the street behind some cars'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'a bathroom that has it's lights turned on'. Find frequency of the letter 'h'
Example Output: 4

Example Input: Sentence: 'a boy in a blue shirt boarding a school bus'. Find frequency of the letter 's'
Example Output:
3