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: 'an airplane as it is taking off and is ascending into space'. How many words start with the letter 'a' in the sentence.
Example Output: 5

Example Input: Sentence: 'a brown and white dog standing on a lush green field'. How many words start with the letter 'b' in the sentence.
Example Output: 1

Example Input: Sentence: 'an electric fence with a sign behind it that says a flood can happen anywhere'. How many words start with the letter 'a' in the sentence.
Example Output:
4