#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <math.h>
#include <stdio.h>
#include <limits.h>
#include <iostream>
#include <algorithm>

#define read(a) scanf("%d", &a)
#define write(a) printf("%d\n", a)
#define file_in(a) freopen(a, "r", stdin)
#define file_out(a) freopen(a, "w", stdout)
#define FOR(a, b) for(int a = 0; a < b; a++)

static const int MAX_N = 1005;
static const int MAX_SIZE = 100005;
static const double pi = 3.1415926535897932;
static const double epsilon = 0.00000000001;

using namespace std;

int n, m;
string bar[MAX_N];

void input()
{
    read(n);
    read(m);
    FOR(i, n)
    {
        cin >> bar[i];
    }
}

bool buy()
{
    for(int i = 0; i < n; i++)
        for(int j = 1; j < n - 1; j++)
            if(bar[j][i] != bar[j - 1][i])
            {
                return false;
            }

    FOR(i, n) if(bar[n - 1][i] < '0' or bar[n - 1][i] > '9') return false;

    return true;
}

int main()
{
    file_in("barcode.in");
    file_out("barcode.out");
    input();

    if(buy()) cout << "BUY\n";
    else cout << "NO\n";
}