while true 
    m, f, r = gets.split.map {|s| s.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
