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

Example Input: Sentence: 'a bird is standing on the ledge of the building'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a man standing in front of a statue near a metal pole'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a cat is relaxing on top of an old television'. Find frequency of the letter 'd'
Example Output:
1