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

Example Input: Sentence: 'a woman holds some baked goods at the convention'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a bird on a tree is flapping its wings'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a person is coming down a hill in the snow on skis'. Find frequency of the letter 'c'
Example Output:
1