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

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

Example Input: Sentence: 'there is a baseball game on and a few people are watching'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'two skiers are passing each other on the slope'. Find frequency of the letter 'r'
Example Output:
3