//******************************
//????n-gram????
//??????
//???1300012712
//???2013?12?11?
//******************************
int main()
{
	char str[520];
	int num[520] = {0};
	char *pc;
	int i, j, k, l, m, n, len;
	cin >> n;
	cin >> str;
	pc = str;  // ???????????
	len = strlen(str);
	int max = -1;
	for (i = 0; i <= len - n; i++)  // ??0?????n???????????
	{
		for (j = i; j <= len - n; j++)  // ???????? ???i????????
		{
			for (k = 0; k < n; k++)
			{
				if (*(pc + i + k) != * (pc + j + k)) break;  // ????????????????? ????
			}
			if (k == n) num[i]++;  // ????????????????
		}
		if (num[i] > max) max = num[i];   // ?????????
	}
	l = 0;
	if (max <= 1)  // ???????????NO
	{
		cout << "NO" << endl;
		return 0;
	}
	cout << max << endl; 
	for (i = 0; i < len - n; i++) // ????????????????? ??????????
	{
		if (num[i] == max) 
		{
			for (j = 0; j < n; j++)
			{
				cout << *(pc + i + j);
			}
			cout << endl;	
		}
	}
	return 0;
}