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

Example Input: Sentence: 'a man hits a tennis ball with a racket on the court'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'a person on a surfboard in the surf of a wave'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'closeup of a glass vase holding red flowers next to meshing'. Find frequency of the letter 'x'
Example Output:
1