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

Example Input: Sentence: 'people playing wii bowling in a modern library'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a yellow fire hydrant in the forefront of a roadway'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'two giraffees enter acting together in a zoo'. Find frequency of the letter 'o'
Example Output:
4