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: 'guy leans on truck's hood while taking pics next to parked motorcycles'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'two girls with three skateboards in the middle of the sidewalk'. Find frequency of the letter 'e'
Example Output:
7