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

Example Input: Sentence: 'two women standing next to each other holding tennis rackets'. Find frequency of the letter 't'
Example Output: 7

Example Input: Sentence: 'statues of 2 zebras and 1 giraffe in a doorway'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a cat is sleeping on the arm of a couch'. Find frequency of the letter 'r'
Example Output:
1