/*
找出当前界面的 x，y 的左右坐标 枚举边界，并且判断界面内是否全为‘.’
*/
#include<cstdio>
#include<cstring>
#include<algorithm>
#define INF 1<<30
using namespace std;
char s[110][110];
int aph[30],n,m,ln;
int judge(int x)
{
int lx,ly,rx,ry;
lx = ly = INF;
rx = ry = -1;
for(int i = 0; i < n; i++)
for(int j = 0; j < m; j++)
{
if(s[i][j]==x+'A')
{
lx = min(lx,i);
ly = min(ly,j);
rx = max(rx,i);
ry = max(ry,j);
}
}
if(lx==INF||ly==INF||rx==-1||ry==-1||rx-lx<2||ry-ly<2)
{
return 0;
}
for(int i = lx; i <= rx; i++)
if(s[i][ly]!=x+'A'||s[i][ry]!=x+'A')
return 0;
for(int i = ly; i <= ry; i++)
if(s[lx][i]!=x+'A'||s[rx][i]!=x+'A')
return 0;
for(int i = lx+1; i < rx; i++)
for(int j = ly+1; j < ry; j++)
if(s[i][j]!='.')
return 0;
return 1;
}
int solve()
{
memset(aph,0,sizeof(aph));
for(int i = 0; i < n; i++)
for(int j = 0; j < m; j++)
if(s[i][j]!='.')
aph[s[i][j]-'A']++;
for(int i = 0; i < 26; i++)
{
if(!aph[i]) continue;
if(judge(i))
printf("%c",i+'A');
}
}
int main()
{
while(scanf("%d%d",&n,&m)==2)
{
if(!n&&!m) break;
for(int i = 0; i < n; i++)
scanf("%s",&s[i]);
solve();
printf("\n");
}
return 0;
}