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