In this task, you need to count the number of words in a sentence that start with the given letter. Answer with numbers and not words.

Example Input: Sentence: 'a young woman is picking a black cat up off of the floor'. How many words start with the letter 'a' in the sentence.
Example Output: 2

Example Input: Sentence: 'a woman in a white tshirt and black pants skiing down a small hill'. How many words start with the letter 'd' in the sentence.
Example Output: 1

Example Input: Sentence: 'a couple of cellphones that are on a rock'. How many words start with the letter 'a' in the sentence.
Example Output:
3