'use strict';
function Main(input) {
  let array = input.split("\n");
  array.shift();
  let sort = array.sort();
  let out = '';
  for (let i = 0; i < sort.length; i++) {
    out += sort[i];
  }
  console.log(`${out}`);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));