import Data.Functor ((<$>))

grade :: Int -> Int -> Int -> String
grade m f r
  | m == -1 || f == -1 = "F"
  | m + f < 30         = "F"
  | m + f < 50         = if r >= 50 then "C" else "D"
  | m + f < 65         = "C"
  | m + f < 80         = "B"
  | otherwise          = "A"

main :: IO ()
main = do
  [m, f, r] <- map read . words <$> getLine
  if m == (-1) && f == (-1) && r == (-1)
    then return ()
    else putStrLn (grade m f r) >> main

