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

Example Input: Sentence: 'a surfer is in the middle of an ocean wave'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a display case filled with lots of different clocks'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a woman sitting in a blue chair on top of a boat'. Find frequency of the letter 't'
Example Output:
4