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

Example Input: Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'two pepperoni pizzas and a cup on a counter'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'the wooden table is covered with plates and bowls of food'. Find frequency of the letter 'o'
Example Output:
7