#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  cin >> a >> b;
  int p;
  string s[100];
  string s1, s2, s3 = "", s4 = "", s5 = "", s6, s7, s8, s9;
  for (int i = 0; i < a; i++) {
    int p = 0;
    cin >> s[i];
    s6 = s[i];
    reverse(s6.begin(), s6.end());
    if (s[i] == s6) {
      s7 = s[i];
    }
  }
  for (int i = 0; i < a - 1; i++) {
    s2 = s[i];
    reverse(s[i].begin(), s[i].end());
    for (int j = i + 1; j < a; j++) {
      if (s[i] == s[j]) {
        s3 = s3 + s2;
        s8 = s[j];
        s4 = s8 + s4;
      }
    }
  }
  s9 = s3 + s7 + s4;
  cout << s9.size() << endl;
  int l = s9.size();
  if (l != 0) {
    cout << s3 + s7 + s4 << endl;
  }
}
