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

Example Input: Sentence: 'two metal benches on an empty train platform'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a woman in a tank top reaches out for a frisbee'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'group of vehicles traveling down a busy city street'. Find frequency of the letter 'g'
Example Output:
2