#include <iostream>
#include <fstream>
#include <map>
#include <algorithm>
#include <set>
#include <string>
#include <vector>
#include <cctype>

using namespace std;
#define myforeach(c,it,ty) for(ty::iterator it=c.begin() ; it!=c.end() ; ++it )
typedef set<char> charset;
struct frame
{
    int x[2];
    int y[2];
    charset lower;
    frame(){};
    frame(int x0, int y0){
        x[0]=x[1]=x0;
        y[0]=y[1]=y0;
    }
    void add(int x1, int y1){
        if ( x1<x[0] ) x[0]=x1;
        if ( x[1]<x1 ) x[1]=x1;
        if ( y1<y[0] ) y[0]=y1;
        if ( y[1]<y1 ) y[1]=y1;
    }
};
typedef map<char,frame> frames;
typedef vector<string> field;

void search(frames &fra, const string &his)
{
    myforeach(fra, cur, frames){
        if ( find(his.begin(), his.end(), cur->first)!=his.end() )
            continue;
        charset &low = cur->second.lower;
        charset::iterator cit=low.begin();
        for ( ; cit!=low.end() &&
              find(his.begin(), his.end(),*cit)!=his.end() ; ++cit ) ;
        if ( cit==low.end() ) {
            string nexthis = his+cur->first;
            if ( nexthis.size() == fra.size() ) {
                cout<<nexthis<<endl;
            }
            else {
                search(fra, nexthis);
            }
        }
    }
}

int main(void)
{
    int height, width;
    ifstream is("frame.txt");
    while ( is>>height>>width && height ) {
        field fl;
        for ( int i=0 ; i<height ; i++ ) {
            string str;
            is>>str;
            fl.push_back(str);
        }
        frames fra;
        for ( int i=0 ; i<height ; i++ ) {
            for ( int j=0 ; j<width ; j++ ) {
                char c=fl[i][j];
                if ( isupper(c) ) {
                    if ( fra.count(c)==0 ) {
                        fra[c]=frame(i,j);
                    }
                    else{
                        fra[c].add(i,j);
                    }
                }
            }
        }
        myforeach(fra, it, frames){
            char my_c=it->first;
            frame &f=it->second;
            for ( int side=0 ; side<2 ; side++ ) {
                for ( int x=f.x[0] ; x<=f.x[1] ; x++ ) {
                    char here=fl[x][f.y[side]];
                    if ( here!=my_c ) {
                        fra[here].lower.insert(my_c);
                    }
                }
                for ( int y=f.y[0] ; y<=f.y[1] ; y++ ) {
                    char here=fl[f.x[side]][y];
                    if ( here!=my_c ) {
                        fra[here].lower.insert(my_c);
                    }
                }
            }
        }
        string str;
        search(fra,str);
        cout<<endl;
    }
}