main = do
  [a,b,c] <- map read . words <$> getLine :: IO [Int]
  if a ^ 2 + b ^ 2 == c ^ 2
    then print $ a * b `div` 2
    else if a ^ 2 + c ^ 2 == b ^ 2
           then print $ a * c `div` 2
           else print $ b * c `div` 2
