#include <iostream>
#include <fstream>

using namespace std;

int N,M;
char b[1000][1000];
bool result;

void loadFromFile(){
    ifstream in;
    in.open("barcode.in");
    in >> N >> M;

    for(int i=0; i<N; i++ )
        for(int j=0; j<M; j++)
            in >> b[i][j];
}



void output(){
    ofstream out;
    out.open ("barcode.out");
    if( result )
        out << "BUY";
    else
        out << "NO";
    out.close();
}

bool calc(){
    char d;
    for(int i=0;i<M;i++){
        d = b[0][i];
        for(int j=1;j<N-1;j++){
            if(b[j][i] != d)
                return false;
        }
        if( b[N-1][i] < '1' || b[N-1][i] > '9' )
            return false;
    }
    return true;
}

int main()
{
    loadFromFile();
    result = calc();
    output();

    return 0;
}