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

Example Input: Sentence: 'a log in the water with three birds perched on it'. Find frequency of the letter 'r'
Example Output: 4

Example Input: Sentence: 'one woman is arranging the other woman's hair'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'lots of food is displayed in dishes on the table'. Find frequency of the letter 's'
Example Output:
5