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 couple of kids are sitting in the grass'. How many words start with the letter 'a' in the sentence.
Example Output: 2

Example Input: Sentence: 'a mother bear and her two young are crossing a shallow river'. How many words start with the letter 'a' in the sentence.
Example Output: 4

Example Input: Sentence: 'a dock with a boat on water between groups of trees'. How many words start with the letter 'a' in the sentence.
Example Output:
2