#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <climits>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <cstdio>
#include <limits>
#include <vector>
#include <cstdlib>
#include <fstream>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
using namespace std;

int n, m;
char barc[1001][1001];

bool shouldBuy()
{
	for (int j = 0; j < m; j++)
	{
		char first = barc[0][j];
		for (int i = 1; i < n - 1; i++)
		{
			if (barc[i][j] != first)
				return false;
		}
	}
	for (int j = 0; j < m; j++) {
		if (barc[n - 1][j] < '0' || barc[n - 1][j] > '9') {
			return false;
		}
	}
	return true;
}

int main() {
	ifstream in("barcode.in");
	ofstream out("barcode.out");

	in >> n >> m;
	for (int i = 0; i < n; i++)
		for (int j = 0; j < m; j++)
			in >> barc[i][j];
	bool toBuy = shouldBuy();
	if (toBuy)
		out << "BUY" << endl;
	else
		out << "NO" << endl;
	return 0;
}