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

Example Input: Sentence: 'a person is water skiing and there are birds flying around above them'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'man sitting on the side of a van playing the guitar'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a horse drawn carriage sitting in the middle of a street'. Find frequency of the letter 'd'
Example Output:
3