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

Example Input: Sentence: 'a boy swinging a bat while playing baseball'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'a person dressed all in black snowboarding down a slope'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a person riding a surf board on a body of water'. Find frequency of the letter 'o'
Example Output:
5