#include <queue>#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>using namespace std;#define maxm 3000001#define INF 0x7fffffffconst int dx[5]={0,-1,0,1,0};const int dy[5]={0,0,1,0,-1};int n,m,d[100][500],tot,s,t,quex[maxm],quey[maxm],qued[maxm];int head[500050],cnt,deep[500050],E[maxm],V[maxm],F[maxm],size,iap;char map[25][25];bool if_[25][25];void bfs(int sx,int sy){    int h=0,tail=1;    memset(if_,false,sizeof(if_));    quex[0]=sx,quey[0]=sy,qued[0]=0,if_[sx][sy]=true;    while(h<tail)    {        int nowx=quex[h],nowy=quey[h],nowd=qued[h++];        for(int i=1;i<=4;i++)        {            int x=nowx+dx[i],y=nowy+dy[i],dd=nowd+1;            if(x>0&&x<=n&&y>0&&y<=m&&map[x][y]==‘.‘&&!if_[x][y])            {                if_[x][y]=true,d[tot][(x-1)*m+y]=dd;                quex[tail]=x,quey[tail]=y,qued[tail++]=dd;            }        }    }}inline void edge_add(int u,int v,int f){    E[++cnt]=head[u],V[cnt]=v,F[cnt]=f,head[u]=cnt;    E[++cnt]=head[v],V[cnt]=u,F[cnt]=0,head[v]=cnt;}bool BFS(){    memset(deep,-1,sizeof(deep));    deep[s]=0;queue<int>que;que.push(s);    while(!que.empty())    {        int now=que.front();que.pop();        for(int i=head[now];i;i=E[i])        {            if(F[i]>0&&deep[V[i]]<0)            {                deep[V[i]]=deep[now]+1;                if(V[i]==t) return true;                que.push(V[i]);            }        }    }    return false;} int flowing(int now,int flow){    if(now==t||flow<=0) return flow;    int oldflow=0;    for(int i=head[now];i;i=E[i])    {        if(F[i]>0&&deep[V[i]]==deep[now]+1)        {            int pos=flowing(V[i],min(flow,F[i]));            flow-=pos,oldflow+=pos;            F[i]-=pos,F[i^1]+=pos;            if(flow==0) return oldflow;        }    }    if(oldflow==0) deep[now]=-1;    return oldflow;}bool check(int k){    memset(head,0,sizeof(head));cnt=1;    for(int i=1;i<=n;i++)    {        for(int j=1;j<=m;j++)        {            if(map[i][j]==‘.‘) edge_add(s,(i-1)*m+j,1);        }    }    for(int i=1;i<=tot;i++)    {        int kop=size*i+tot;        edge_add(size+i,t,k);        for(int j=1;j<=k;j++)        {            if(j!=1) edge_add(kop+j-1,kop+j,INF);            edge_add(kop+j,size+i,1);        }        for(int j=1;j<=size;j++) if(d[i][j]<=k) edge_add(j,kop+d[i][j],1);    }    int pos=0;    while(BFS())    pos+=flowing(s,INF);    if(pos>=iap) return true;    else return false;}int main(){    scanf("%d%d",&n,&m);    memset(d,127/3,sizeof(d));    for(int i=1;i<=n;i++) scanf("%s",map[i]+1);    for(int i=1;i<=n;i++)    {        for(int j=1;j<=m;j++)        {            if(map[i][j]==‘D‘)            {                tot++;                bfs(i,j);            }            if(map[i][j]==‘.‘) iap++;        }    }    t=500049,size=n*m;    int l=0,r=size,ans=-1;    while(l<=r)    {        int mid=l+r>>1;        if(check(mid)) ans=mid,r=mid-1;        else l=mid+1;    }    if(ans==-1) cout<<"impossible";    else cout<<ans;    return 0;}