How is yellowstone different from other volcanoes?

The Yellowstone caldera differs from most other volcanoes in that it is classified with a Volcanic Explosivity Index(VEI) of 8. What this means is that the volcano can potentially have a volcanic ejecta mass of over 1,000,000,000,000 tons. Now compare this to the VEI of some other volcanoes:
Eyjafjallajökull: 4
Mt. St. Helens: 5
Krakatoa: 6
Tambora: 7

There is only a small number of known VEI 8 eruptions, only around 42 in the last 36 million years with the last known one being 25,360 years ago. In other words, Yellowstone is really dangerous and should be monitored closely.

Answer( genes );

F=31 because you subtract 39-8