#include <iostream>
#include <fstream>
#include <climits>
#include <algorithm>

using namespace std;

#define cin fin
ifstream fin("frame.txt");


#define SIZE_MAX 30

int height, width;
char B[SIZE_MAX][SIZE_MAX];

int rmin[26], rmax[26], cmin[26], cmax[26];

int nframe;
int F[26];
int W[26];

void backtrack( int curr, int used, int order[] ) {
	if ( curr == nframe ) {
		for ( int i = 0; i < nframe; i++ )
			cout << (char)('A' + order[i]);
		cout << endl;
		return;
	}
	
	for ( int n = 0; n < nframe; n++ ) {
		if ( used&(1<<F[n]) ) continue;
		if ( (used&W[F[n]]) != W[F[n]] ) continue;
		order[curr] = F[n];
		backtrack(curr+1, used|(1<<F[n]), order);
	}
}

void compute() {
	int order[26];
	backtrack(0, 0, order);
	cout << endl;
}

int main() {
	while ( true ) {
		cin >> height >> width;
		if ( height == 0 && width == 0 ) break;

		for ( int r = height-1; r >= 0; r-- )
			for ( int c = 0; c < width; c++ ) cin >> B[r][c];

		for ( int i = 0; i < 26; i++ )
			rmin[i] = INT_MAX, rmax[i] = INT_MIN, cmin[i] = INT_MAX, cmax[i] = INT_MIN;

		for ( int r = 0; r < height; r++ )
			for ( int c = 0; c < width; c++ ) {
				if ( B[r][c] == '.' ) continue;

				const int ch = B[r][c]-'A';
				rmin[ch] = min(rmin[ch], r);
				rmax[ch] = max(rmax[ch], r);
				cmin[ch] = min(cmin[ch], c);
				cmax[ch] = max(cmax[ch], c);
			}

		nframe = 0;
		for ( int i = 0; i < 26; i++ )
			if ( rmin[i] != INT_MAX ) F[nframe++] = i;

		for ( int i = 0; i < 26; i++ ) W[i] = 0;

		for ( int i = 0; i < nframe; i++ ) {
			const int self = F[i];

			for ( int r = rmin[self]; r <= rmax[self]; r++ )
				for ( int c = cmin[self]; c <= cmax[self]; c++ ) {
					if ( r > rmin[self] && r < rmax[self] && c > cmin[self] && c < cmax[self] ) continue;
					if ( B[r][c] == (self+'A') ) continue;
					const int opp = B[r][c] - 'A';
					W[opp] |= (1 << self);
				}
		}

		compute();
	}

	return 0;
}