#include <iostream>
#include <fstream>
using namespace std;

int main() {

	fstream myfile;
	myfile.open("barcode.in");
	ofstream wfile("barcode.out");

	int row;
	int col;
	
	char first[1000];
	char second[1000];

	char output;
	
	if (myfile.is_open()) {
		while (!myfile.eof()) {


			myfile >> row;
			myfile >> col;

			for (int j = 0; j < col; j++)
			{
				myfile >> first[j];
			}

			for (int i = 1; i < row-1; i++)
			{
				for (int j = 0; j < col; j++)
				{
					myfile >> output;

					if (first[j] == output)
					{
						first[j] = output;
					}
					else
					{
						if (wfile.is_open())
						{
							wfile << "NO";
							wfile.close();
						}
						return 0;
					}
				}
				if (wfile.is_open())
				{
					wfile << "BUY";
					wfile.close();
				}
				return 0;
			}

		}
	}
	myfile.close();

}