#include <bits/stdc++.h>

#define INF (1 << 30)
#define LLINF (1LL << 62)
#define mod 666013

using namespace std;

int n, m, i, j;
int sum[1005];
char s[1005];

int main()
{
    freopen("barcode.in", "r", stdin);
    freopen("barcode.out", "w", stdout);

    scanf("%d%d\n", &n, &m);
    for(i = 1; i < n; i++)
    {
        gets(s + 1);
        for(j = 1; j <= m; j++)
            sum[j] += (s[j] == '#');
    }

    for(j = 1; j <= m; j++)
        if(sum[j] != 0 && sum[j] != n - 1)
        {
            puts("NO");
            return 0;
        }
    puts("BUY");

    return 0;
}