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

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: 'the two dogs are playing in the pen together'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a large delta passenger jet being prepared for take off from an airport runway'. Find frequency of the letter 'o'
Example Output:
4