"use strict";
const main = arg => {
  let N = arg.split("\n")[0].split(" ")[0];
  let rows = arg.split("\n").slice(1, N+1);
  let sortRows = rows.sort();
  let joinSortRows = sortRows.join("");
  console.log(joinSortRows);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));
