N=gets.to_i
ss=STDIN.read.split.map{|s| s + "~"}
result=""
while ! ss.empty? do
    ss.sort!
    result += ss[0][0]
    ss[0] = ss[0][1,10000]
    if(ss[0].length==1)
        ss.slice!(0,1)
    end
end
puts result