program p1128;
const
   inf='';
   ouf='';
type
   str=string;
var
  bool:array['A'..'Z']of boolean;
  a:array[0..200,0..200]of char;
  b:array['A'..'Z']of record
                          x,y,w,z:longint;
                       end;
  line:array['A'..'Z']of longint;
  map:array['A'..'Z','A'..'Z']of longint;
  s:str;
  n,m,i,j,k,l,top,total:longint;
procedure pre;
var
  i,j:longint;
  ch,ch2:char;
begin
   for i:=1 to n do
     for j:=1 to m do
       if a[i,j] in ['A'..'Z'] then
         begin
            if (b[a[i,j]].x=0)or(b[a[i,j]].x>i) then b[a[i,j]].x:=i;
            if (b[a[i,j]].y=0)or(b[a[i,j]].y>j) then b[a[i,j]].y:=j;
            if (b[a[i,j]].w=0)or(b[a[i,j]].w<i) then b[a[i,j]].w:=i;
            if (b[a[i,j]].z=0)or(b[a[i,j]].z<j) then b[a[i,j]].z:=j;
         end;
   for ch:='A' to 'Z' do
     if not((b[ch].x=0)and(b[ch].y=0)and(b[ch].w=0)and(b[ch].z=0)) then
      begin
           for i:=b[ch].x to b[ch].w do
            begin
             if a[i,b[ch].y]<>ch then map[ch,a[i,b[ch].y]]:=1;
             if a[i,b[ch].z]<>ch then map[ch,a[i,b[ch].z]]:=1;
            end;
           for i:=b[ch].y to b[ch].z do
            begin
             if a[b[ch].x,i]<>ch then map[ch,a[b[ch].x,i]]:=1;
             if a[b[ch].w,i]<>ch then map[ch,a[b[ch].w,i]]:=1;
            end;
      end;
   for ch:='A' to 'Z' do
      for ch2:='A' to 'Z' do
          if map[ch2,ch]<>0 then inc(line[ch]);
end;
procedure make(i:longint);
var
  top:longint;
  ch,ch2:char;
begin
     for ch:='A' to 'Z' do
       if (line[ch]=0)and(bool[ch]=false) then
          begin
               top:=0;
               for ch2:='A' to 'Z' do
                  if map[ch,ch2]<>0 then
                    begin
                       inc(top);
                       dec(line[ch2]);
                       map[ch,ch2]:=-1;
                    end;
               s:=s+ch;
               bool[ch]:=true;
               if i=total then
                  writeln(s)
                          else make(i+1);
               bool[ch]:=false;
               delete(s,length(s),1);
               for ch2:='A' to 'Z' do
                 if map[ch,ch2]=-1 then
                   begin
                       inc(line[ch2]);
                       map[ch,ch2]:=1;
                   end;
          end;

end;
procedure pre2;
var
  ch:char;
begin
   for ch:='A' to 'Z' do
      if bool[ch] then line[ch]:=0
                  else line[ch]:=-1;
end;
begin
  assign(input,inf);assign(output,ouf);
  reset(input);rewrite(output);

  readln(n);
  readln(m);
  total:=0;
  s:='';
  top:=0;
  fillchar(bool,sizeof(bool),false);
  fillchar(b,sizeof(b),0);
  fillchar(map,sizeof(map),0);
  for i:=1 to n do
     begin
     for j:=1 to m do
         begin
           read(a[i,j]);
           if (bool[a[i,j]]=false)and(a[i,j] in ['A'..'Z']) then
             begin
                 bool[a[i,j]]:=true;
                 total:=total+1;
             end;
         end;
     readln;
     end;
  pre2;
  fillchar(bool,sizeof(bool),false);
  pre;
  make(1);

  close(input);close(output);
end.