while (m,f,r = gets.split(" ").map(&:to_i)) != [-1,-1,-1]
    if m + f < 30 or m < 0 or f < 0
        puts "F"
    elsif m + f >= 80
        puts "A"
    elsif m + f >= 65
        puts "B"
    elsif m + f >= 50 or r >=50
        puts "C"
    else
        puts "D"
    end
end


