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

Example Input: Sentence: 'a person dressed all in black snowboarding down a slope'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'two laptop computers sitting on top of a white table'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a group of men in a field playing baseball'. Find frequency of the letter 'u'
Example Output:
1