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

Example Input: Sentence: 'a group of cows that are standing in the grass'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'many white birds flying above a river that flows in front of a forest'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a chubby cat sits under a parked car in the shade'. Find frequency of the letter 'a'
Example Output:
6