#include <bits/stdc++.h>

using namespace std;

bool palindrom(string s)
{
    int l = 0, r = s.size() - 1;
    bool ans = 1;
    while (l < r)
    {
        if (s [l] != s [r])
            ans = 0;
        l++;
        r--;
    }
    return ans;
}

int main()
{
    int n, m;
    cin >> n >> m;
    string s [n];
    for (int i = 0; i < n; i++)
        cin >> s [i];
    bool visited [n] = {};
    map <string, int> have;
    deque <string> ans;
    for (int i = 0; i < n; i++)
    {
        reverse(s [i].begin(), s [i].end());
        if (have [s [i]] != 0)
        {
            visited [have [s [i]] - 1] = 1;
            have [s [i]] = 0;
            ans.push_front(s [i]);
            reverse(s [i].begin(), s [i].end());
            ans.push_back(s [i]);
            visited [i] = 1;

        }
        else
        {
            reverse(s [i].begin(), s [i].end());
            have [s [i]] = i + 1;
        }
    }
    int sz = ans.size() * m, it = -1;
    for (int i = 0; i < n; i++)
    {
        if (!visited [i])
        {
            if (palindrom(s [i]))
            {
                sz += m;
                it = i;
                break;
            }
        }
    }
    cout << sz << "\n";
    if (it == -1)
    {
        for (int i = 0; i < ans.size(); i++)
            cout << ans [i];
    }
    else
    {
        for (int i = 0; i < ans.size() / 2; i++)
            cout << ans [i];
        cout << s [it];
        for (int i = ans.size() / 2; i < ans.size(); i++)
            cout << ans [i];
    }
}