#include<iostream>
#include<fstream>
using namespace std ;
bool isValid(string a)
{
    for(int i=0; i<a.size()-1; i++)
    {
        if(!(a[i]=='#' or a[i]=='.'))
        {
            return false;
        }
    }
    return true;
}
int main (  )
{
    ifstream read("barcode.in");
    ofstream write("barcode.out");
    int n,m;
    read>>n>>m;
    string a;
    read>>a;
    string b;
    bool buy=true;
    if(a.size()!=m or !isValid(a))
    {
        buy=false;
    }
    for(int i=0; i<n-2; i++)
    {
        read>>b;
        if(b!=a or !buy)
        {
            buy=false;
        }
    }
    read>>b;
    if(buy and b.size()==m)
    {
        write<<"BUY";
    }
    else
    {
        write<<"NO";
    }
    write<<endl;
    return 0 ;
}