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

using namespace std;

static bool table[26][26];
static char s[27];
static int  d[26];

void tsort(int n, int k)
{
	if(k == n) {
		s[n] = '\0';
		cout << s << endl;
		return;
	}

	for(int i = 0; i < 26; i++) {
		if(d[i] == 1) {
			s[k] = (char)(i + 'A');
			for(int j = 0; j < 26; j++) if(table[j][i]) --d[j];
			tsort(n, k + 1);
			for(int j = 0; j < 26; j++) if(table[j][i]) ++d[j];
		}
	}
}

int main(void)
{
	ifstream cin("frame.txt");
	int state[30][30];
	int xmin[26], xmax[26], ymin[26], ymax[26];

	int h, w;
	while(cin >> h >> w) {
		if(h == 0 && w == 0) break;

		for(int i = 0; i < 26; i++) {
			xmin[i] = w;
			xmax[i] = 0;
			ymin[i] = h;
			ymax[i] = 0;
		}

		for(int y = 0; y < h; y++) {
			for(int x = 0; x < w; x++) {
				char c;
				cin >> c;
				int m = c - 'A';
				state[x][y] = m;
				if(c == '.') continue;
				if(x < xmin[m]) xmin[m] = x;
				if(x > xmax[m]) xmax[m] = x;
				if(y < ymin[m]) ymin[m] = y;
				if(y > ymax[m]) ymax[m] = y;
			}
		}

		for(int i = 0; i < 26; i++) {
			for(int j = 0; j < 26; j++) {
				table[i][j] = false;
			}
		}

		for(int i = 0; i < 26; i++) {
			if(xmin[i] > xmax[i]) continue;

			for(int x = xmin[i]; x <= xmax[i]; x++) {
				table[state[x][ymin[i]]][i] = true;
				table[state[x][ymax[i]]][i] = true;
			}

			for(int y = ymin[i]; y <= ymax[i]; y++) {
				table[state[xmin[i]][y]][i] = true;
				table[state[xmax[i]][y]][i] = true;
			}
		}

		int n = 0;

		for(int i = 0; i < 26; i++) {
			if(xmin[i] > xmax[i]) {
				d[i] = -1;
			}
			else {
				d[i] = 0;
				// Note that table[i][i] is always true
				for(int j = 0; j < 26; j++) if(table[i][j]) ++d[i];
				++n;
			}
		}

		tsort(n, 0);
		cout << endl;
	}

	return 0;
}