#include <iostream>
#include <iomanip>
#include <vector>
#include <string>
#include <queue>
#include <map>
#include <set>
#include <string>
#include <algorithm>
#include <cmath>
#include <unordered_map>

using namespace std;
#define ll long long
#define nl endl
#define cy cout << "Yes\n"
#define cn cout << "No\n"
#define sz s.size()

void sherry()
{
    ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}
/*
ll gcd(ll a, ll b)
{
    while (b)
    {
        a %= b;
        swap(a, b);
        c++;
    }
    return a;
}
ll lcm(ll a, ll b)
{
    return a * b / gcd(a, b);
}*/

bool is_prime(ll x)
{
    if (x == 2 || x == 3 || x == 1)
        return true;
    if (x % 2 == 0)
        return false;
    for (ll i = 3; i <= sqrt(x); i += 2)
    {
        if (x % i == 0)
            return false;
    }
    return true;
}
/*
int s[1000001];
int divisors(int n)
{
    ll c = 0;
    if (s[n] != 0)
        return s[n];
    for (int i = 1; i * i <= n; i++)
    {
        if (n % i == 0)
        {
            c++;
            if (i * i != n)
                c++;
        }
    }
    s[n] = c;
    return c;
}*/
/*
bool comp(const pair<string, long double> &a, const pair<string, long double> &b)
{
    if (a.second == b.second)
    {
        return a.first < b.first;
    }
    return a.second > b.second;
}*/
string s[100];
int main()
{
    sherry();
    set<string> dict;
    int n, m, i;
    cin >> n >> m;
    for (i = 0; i < n; i++)
    {
        cin >> s[i];
        dict.insert(s[i]);
    }
    vector<string> left, right;
    string mid;
    for (i = 0; i < n; i++)
    {
        string t = s[i];
        reverse(t.begin(), t.end());
        if (t == s[i])
            mid = t;
        else if (dict.find(t) != dict.end())
        {
            left.push_back(s[i]);
            right.push_back(t);
            dict.erase(s[i]);
            dict.erase(t);
        }
    }
    cout << left.size() * m * 2 + mid.size() << endl;
    for (string x : left)
        cout << x;
    cout << mid;
    reverse(right.begin(), right.end());
    for (string x : right)
        cout << x;
    cout << endl;
}