"use strict";

function Main(input) {
  input = input.split("\n");
  const tmp = input[0].split(" ")
  var N = parseInt(tmp[0]);
  var L = parseInt(tmp[1]);
  input.shift();
  var S = input;
  S.sort();
  console.log(S.join(''));
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));
