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

Example Input: Sentence: 'men standing in the grass with a frisbee'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'a picture of two computers on a table'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'large pile of blood oranges displayed in photo'. Find frequency of the letter 'o'
Example Output:
6