#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m;
  cin >> n >> m;
  vector<string> v;
  vector<string> res;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    v.push_back(s);
  }
  bool flag = 0;
  string t = "", pal = "";
  for (int i = 0; i < v.size(); i++) {
    string s = v[i];
    t = s;
    reverse(s.begin(), s.end());
    auto it = find(v.begin(), v.end(), s);
    if (it != v.end()) {
      if (*it == t) {
        flag = 1;
        pal = t;
        continue;
      }
      res.push_back(v[i]);
      v.erase(it);
    }
  }
  string out = "";
  for (auto x : res) {
    out += x;
  }
  string p = out;
  if (flag) out += pal;
  reverse(p.begin(), p.end());
  out += p;
  cout << out.length() << "\n";
  cout << out;
}
