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 brown and white cat laying on blanket on door of open oven'. How many words start with the letter 'o' in the sentence.
Example Output: 5

Example Input: Sentence: 'a silver blue and red jet is flying over a fence'. How many words start with the letter 'f' in the sentence.
Example Output: 2

Example Input: Sentence: 'a little girl in a dress standing in front of a christmas tree'. How many words start with the letter 'a' in the sentence.
Example Output:
3