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

Example Input: Sentence: 'guy leans on truck's hood while taking pics next to parked motorcycles'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a couple of people that are on a baseball field'. Find frequency of the letter 'c'
Example Output: 1

Example Input: Sentence: 'a couple of horses standing near a road'. Find frequency of the letter 'h'
Example Output:
1