m,f,r = gets.split.map(&:to_i)

while true
  exit if m == -1 &&  f == -1 &&  r == -1

  if m == -1 ||  f == -1
    puts "F"
  elsif m+f >= 80
    puts "A"
  elsif m+f >= 65 && m+f < 80
    puts "B"
  elsif m+f >= 50 && m+f < 65
    puts "C"
  elsif m+f >= 30 && m+f < 50 && r >= 50
    puts "C"
  elsif m+f >= 30 && m+f < 50
    puts "D"
  elsif m+f < 30
    puts "F"
  end
  m,f,r = gets.split.map(&:to_i)
end
