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

Example Input: Sentence: 'this is a sign in front of a large cathedral'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'man in air skateboarding on street with building'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a number of people riding horses with people in stands'. Find frequency of the letter 'd'
Example Output:
2