Task: Question Understanding
Input (English): Passage: There is some disagreement on which country has the largest proven gas reserves. Sources that consider that Russia has by far the largest proven reserves include the US CIA (47 600 km³), the US Energy Information Administration (47 800 km³), and OPEC (48 700 km³). However, BP credits Russia with only 32 900 km³, which would place it in second place, slightly behind Iran (33 100 to 33 800 km³, depending on the source).  With Gazprom, Russia is frequently the worlds largest natural gas extractor. Major proven resources (in cubic kilometers) are world 187 300 (2013), Iran 33 600 (2013), Russia 32 900 (2013), Qatar 25 100 (2013), Turkmenistan 17 500 (2013) and the United States 8500 (2013).
Question: How many more cubic kilometers of gas reserves does Iran have compared to the United States?
Output (English): number