#http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ITP1_7_A

while true
    m,f,r = gets.split.map(&:to_i)
    break if(m+f+r==-3)
    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