N = gets.to_i
NGWords = Array.new(N) { gets.chomp }.uniq

matching = [* 0 ... NGWords.size]
ans = ""
N.times do |j|
    a = []
    b = []
    matching.each do |i|
        if NGWords[i][j] == 'a'
            a << i
        else
            b << i
        end
    end
    if a.size <= b.size
        matching = a
        ans << 'a'
    else
        matching = b
        ans << 'b'
    end
end

puts ans