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

Input: Consider Input: Sentence: 'a crowd of people standing in front of a green train'. Find frequency of the letter 'g'

Output: 2

Input: Consider Input: Sentence: 'salad and vegetables with me on top of plate'. Find frequency of the letter 'o'

Output: 3

Input: Consider Input: Sentence: 'a man riding a skateboard down the middle of a street'. Find frequency of the letter 'a'
Output: 6