while true
    m,f,r = gets.chomp.split(" ").map{|item| item.to_i}
    if m==-1 && f==-1 && r==-1
        break
    end
    
    if m==-1 || f==-1
        puts "F"
    elsif m+f>=80
        puts "A"
    elsif m+f>=65
        puts "B"
    elsif m+f>=50
        puts "C"
    elsif m+f>=30
        if r>=50
            puts "C"
        else
            puts "D"
        end
    else
        puts "F"
    end
end