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

Example Input: Sentence: 'a surfer is in the middle of an ocean wave'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a sheep in a field in front of other sheep'. Find frequency of the letter 'f'
Example Output: 3

Example Input: Sentence: 'a giraffe behind a fence in a zoo'. Find frequency of the letter 'e'
Example Output:
4