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