def ascan; gets.split.map(&:to_i); end

strs = gets.to_i.times.map{(gets.chomp+'~').chars}

ans = ''
while !strs.empty?
    item, i = strs.each_with_index.min
    ans << strs[i].shift
    strs.delete_at(i) if strs[i].empty?
end

puts ans.gsub('~','')