//************************************************************************
//*???3.cpp                                                         
//*???n-gram????                                                        
//*??????                                                          
//*???2013?12?12?                                                   
//************************************************************************
int main()
{
	int n , i, j, k, num[510] = {0}, max = 1;       //????num???????????
	char str[510], put[510][8];						 //??put?????????
	cin >> n;
	cin.get();										 //??n???\0
	cin.getline(str, 510);
	int len = strlen(str);
	for(i = 0; i <= len - n; i ++ )
	{	
		k = 0;
		for(j = i ; j < i + n; j++)
		{
			put[i][k] = str[j];				
			k++;
		} 
		put[i][k] = '\0';
	}
	for(i = 0; i <= len - n; i++)
	{
		for(j = i; j <= len - n; j++)
		{			
			if(strcmp(put[i] , put[j]) == 0)
			{				
				num[i]++;
			}
		}
	}
	for(i = 0; i < len - n; i++)
	{
		
		if(num[i] > max)
			max = num[i];
	}
	if(max == 1)
		cout << "NO" << endl;
	else
	{
		cout << max << endl;
		for(i = 0; i < len - n; i++)
		{
			if(num[i] == max)
			{
				cout << put[i] << endl;
					
			}
		}
	}
	
	return 0;
}
	
