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

Example Input: Sentence: 'a woman is looking at sheep off in a field'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'two computers and a laptop set up on a desk'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'an airport with one plane flying away and the other sitting on the runway'. Find frequency of the letter 'o'
Example Output:
4