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

Example Input: Sentence: 'a person on a sidewalk jumping with an umbrella'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a young boy bouncing a tennis ball on a tennis racquet'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'various food sitting on the top of a gas stove'. Find frequency of the letter 'o'
Example Output:
7