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

Example Input: Sentence: 'a bird on a tree is flapping its wings'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'some food on plates next to each other on a table'. Find frequency of the letter 'e'
Example Output: 6

Example Input: Sentence: 'a man sits on a living room sofa with his arms crossed'. Find frequency of the letter 'a'
Example Output:
5