#include <iostream>
#include <cstdio>
using namespace std;
int n,m;
string str[1000];
int main()
{
    freopen("barcode.in","r",stdin);
    freopen("barcode.out","w",stdout);
    cin>>n>>m;
    for(int i=0;i<n;i++)
        cin>>str[i];
    for(int i=0;i<m;i++)
    {
        for(int j=1;j<n;j++)
            if(str[j][i]!=str[j-1][i]&&!(str[j][i]<='9'&&str[j][i]>='0'))
            {
                cout<<"NO"<<endl;
                return 0;
            }
    }
    cout<<"BUY"<<endl;
}