#include<iostream>
#include<fstream>
#include<vector>
#include<algorithm>

using namespace std;

int n;
string a;

char start;
string add;
vector <string> answer;

int maxLen = 0, current;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);

    ifstream cin("password.in");
    ofstream cout("password.out");

    cin >> n;
    cin >> a;

    for(int i = 0;i<n;)
    {
        start = a[i];

        current = 0;
        while(i<n && a[i]==start)
        {
            current++;
            i++;
        }

        maxLen = max(maxLen, current);
    }

    for(int i = 0;i<n;)
    {
        add = "";
        start = a[i];

        current = 0;
        while(i<n && a[i]==start)
        {
            add += a[i];

            current++;
            i++;
        }

        if(current==maxLen) answer.push_back(add);
    }

    cout << answer.size() << '\n';
    sort(answer.begin(), answer.end());

    for(int i = 0;i<answer.size();i++)
        cout << answer[i] << '\n';
}