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

Example Input: Sentence: 'a man swinging a bat while playing baseball'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a white bathroom sink sitting next to a toilet'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'there is a man surfing along in the water'. Find frequency of the letter 'e'
Example Output:
4