while (m, f, r = gets.split.map(&:to_i)) != [-1, -1, -1]
    m = f = 0 if m == -1 || f == -1
    case m + f
    when 80..100
        puts "A"
    when 65...80
        puts "B"
    when 50...65
        puts "C"
    when 30...50
        puts r >= 50 ? "C" : "D"
    else
        puts "F"
    end
end