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

Example Input: Sentence: 'a red building with a crowd of people outside it near the water'. Find frequency of the letter 'e'
Example Output: 7

Example Input: Sentence: 'a man pushing a cart full of bananas'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'two cats lay down on blankets on a bed together'. Find frequency of the letter 'o'
Example Output:
5