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

Example Input: Sentence: 'a crowd of people skiing and enjoying the snow'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a stop sign next to houses with christmas lights'. Find frequency of the letter 's'
Example Output: 7

Example Input: Sentence: 'two men are playing a tennis game on a court'. Find frequency of the letter 'r'
Example Output:
2