#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
 
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif
 
// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
 
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
 
using namespace std;
 
#define ms(s, n) memset(s, n, sizeof(s))
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define FORd(i, a, b) for (int i = (a) - 1; i >= (b); i--)
#define FORall(it, a) for (__typeof((a).begin()) it = (a).begin(); it != (a).end(); it++)
#define sz(a) int((a).size())
#define all(a) (a).begin(), (a).end()
#define uni(a) (a).erase(unique(all(a)), (a).end())
#define pb push_back
#define pf push_front
#define mp make_pair
#define fi first
#define se second
#define prec(n) fixed<<setprecision(n)
#define bit(n, i) (((n) >> (i)) & 1)
#define bitcount(n) __builtin_popcount(n)
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pi;
typedef vector<int> vi;
typedef vector<pi> vii;
const int MOD = (int) 1e9 + 7;
const int INF = (int) 1e9;
const ll LINF = (ll) 1e18;
const ld PI = acos((ld) -1);
const ld EPS = 1e-9;
ll gcd(ll a, ll b) {if(a<b)swap(a,b);ll r; while (b) {r = a % b; a = b; b = r;} return a;}
ll lcm(ll a, ll b) {return a / gcd(a, b) * b;}
ll fpow(ll n, ll k, int p = MOD) {ll r = 1;for (; k; k >>= 1) {if (k & 1) r = r * n % p; n = n * n % p;} return r;}
template<class T> void setmin(T& a, T val) {if (a > val) a = val;}
template<class T> void setmax(T& a, T val) {if (a < val) a = val;}
void addmod(int& a, int val, int p = MOD) {if ((a = (a + val)) >= p) a -= p;}
void submod(int& a, int val, int p = MOD) {if ((a = (a - val)) < 0) a += p;}
int mult(int a, int b, int p = MOD) {return (ll) a * b % p;}
int inv(int a, int p = MOD) {return fpow(a, p - 2, p);}

string mulstring(string s, int count){
	string r = "";
	FOR(i,0,count)
		r += s;
	return r;
}

string rev(string s){
	string r(s);
	reverse(r.begin(), r.end());
	return r;
}

int main(){
	int n, m;
	cin >> n >>m;
	unordered_map<string, int> freq;
	FOR(i,0,n){
		string s;
		cin >> s;
		freq[s]++;
	}
	string prefix = "";
	string middle = "";
	for(auto x: freq){
		string s = x.first;
		int count_s = x.second;
		string r = rev(s);
		if(s==r){
			prefix += mulstring(s,(count_s/2));
			if(middle=="" && count_s%2)
				middle = s;
		}
		else {
			int count_r = freq[r];
			// cout << r << " " << count_r << endl;
			prefix += mulstring(s, min(count_s, count_r));
		}
		freq[s] = 0;
		freq[r] = 0;
	}
	string suffix = rev(prefix);
	string ans = prefix + middle + suffix;
	cout << ans.size() << "\n" << ans << "\n";
	return 0;
}