str = STDIN.read
lines = str.split(/\n/)
lines.shift

output = ""
while true do
        if lines.length == 0 then
                break
        end
        lines.sort!
        output += lines[0][0]
        new_str = lines[0][1..lines[0].length]
        if new_str == "" then
                lines.delete_at(0)
        else
                lines[0] = new_str
        end
end
puts(output)