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

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

Example Input: Sentence: 'a man in his car eating a donut'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a guy on a skate board high in the air'. Find frequency of the letter 'a'
Example Output:
5