#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstdio>

using namespace std;
int n, m, ok;
string a[1024];
int main()
{
    freopen ("barcode.in", "r", stdin);
    freopen ("barcode.out", "w" , stdout);
    cin>>n>>m;
    for (int i=1; i<=n; i++)
    {
        cin>>a[i];
        if (i>1 && i<n)
            if (a[i]!=a[i-1])
            {
                ok=1;
            }
    }
    if (ok==1)
    {
        printf("NO\n");
        return 0;
    }
    for (int i=0; i<m; i++)
    {
        if (a[n][i]<'0' || a[n][i]>'9')
        {
            printf("NO\n");
            return 0;
        }
    }


    printf("BUY\n");
    return 0;
}