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.
Sentence: 'a street view  left shows a curb  with a yellow fire hydrant and a big bus holding a bicycle in front and to the right an island with cars moving on either side and far off street signs a crosswalk with pedestrians and buildings'. How many words start with the letter 'b' in the sentence.