import std.algorithm, std.array, std.container, std.range, std.bitmanip;
import std.numeric, std.math, std.bigint, std.random, core.bitop;
import std.string, std.regex, std.conv, std.stdio, std.typecons;

void main()
{
  auto n = readln.chomp.to!size_t;
  auto si = iota(n).map!(_ => readln.chomp ~ "|").array;

  auto hi = si.heapify!("a > b");
  auto s = "";

  while (hi.front[0] != '|') {
    auto a = hi.front;
    s ~= a[0];
    hi.replaceFront(a[1..$]);
  }

  writeln(s);
}