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

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 black horse with a white dot on its forehead'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a very cute zebra in a big grassy field'. Find frequency of the letter 'b'
Example Output:
2