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 young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'two women sitting on the floor playing video games'. Find frequency of the letter 't'
Example Output:
4