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

Example Input: Sentence: 'an audience watches as a bird spreads its wings'. Find frequency of the letter 'd'
Example Output: 3

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 multi colored dog jumping up to catch a frisbee'. Find frequency of the letter 'a'
Example Output:
3