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

Example Input: Sentence: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a person does a trick on a ramp on a snowboard'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: '4 fighter jets with two women and a cat in the background'. Find frequency of the letter 't'
Example Output:
6