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 person standing on a green field next to a river flying a kite'. How many words start with the letter 'a' in the sentence.
Example Output: 4

Example Input: Sentence: 'a young man wearing glasses with a tooth brush in his mouth'. How many words start with the letter 'b' in the sentence.
Example Output: 1

Example Input: Sentence: 'an old computer is on a wood desk'. How many words start with the letter 'a' in the sentence.
Example Output:
2