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

Example Input: Sentence: 'two men in black wet suits beside a white surfboard'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a bird is perched on a container in a person's hand'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'a plate that has a few small pieces of pizza left'. Find frequency of the letter 's'
Example Output:
3