#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main()
{
    ll m, s;
    cin >> m >> s;
    vector<vector<string>> z1(m, vector<string>(s, "0"));

    for (int i = 0; i < m; i++)
    {

        string a;
        cin >> a;
        z1.at(i).at(0) = a;
        for (int j = 1; j < s; j++)
        {
            string b;
            cin >> b;
            b = z1.at(i).at(j - 1) + b;
            sort(b.begin(), b.end());
            z1.at(i).at(j) = b;
        }
    }
    ll ns;
    for (int i = 0; i < s; i++)
    {
        bool equal = true;
        for (int j = 0; j < m - 1; j++)
        {
            if (z1.at(j).at(i) != z1.at(j + 1).at(i))
            {
                equal = false;
                break;
            }
        }
        if (equal == true)
        {
            ns = i + 1;
            break;
        }
    }
    cout << ns << endl;
    for (int i = 0; i < ns; i++)
    {
        cout << z1.at(0).at(ns - 1).at(i) << " ";
    }
    cout << endl;
    return 0;
}