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

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

Ex Output:
4

Ex Input:
Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'

Ex Output:
4

Ex Input:
Sentence: 'a large clock mounted on the side of a red wall over a fire place'. Find frequency of the letter 'r'

Ex Output:
4