#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
using namespace std;
#define INF 2147483647
#define MAXN 411
#define MAXM 350001
int v[MAXM],cap[MAXM],en,first[MAXN],next[MAXM];
int d[MAXN],cur[MAXN];
queue<int>q;
int n,S,T,N,M;
void Init_Dinic(){memset(first,-1,sizeof(first)); en=0; S=0; n=N*M+2; T=N*M+1;}
void AddEdge(const int &U,const int &V,const int &W)
{v[en]=V; cap[en]=W; next[en]=first[U]; first[U]=en++;
v[en]=U; next[en]=first[V]; first[V]=en++;}
bool bfs()
{memset(d,-1,sizeof(d)); q.push(S); d[S]=0;while(!q.empty()){int U=q.front(); q.pop();for(int i=first[U];i!=-1;i=next[i])if(d[v[i]]==-1 && cap[i]){d[v[i]]=d[U]+1;q.push(v[i]);}}return d[T]!=-1;
}
int dfs(int U,int a)
{if(U==T || !a) return a;int Flow=0,f;for(int &i=cur[U];i!=-1;i=next[i])if(d[U]+1==d[v[i]] && (f=dfs(v[i],min(a,cap[i])))){cap[i]-=f; cap[i^1]+=f;Flow+=f; a-=f; if(!a) break;}if(!Flow) d[U]=-1;return Flow;
}
int max_flow()
{int Flow=0,tmp=0;while(bfs()){memcpy(cur,first,(n+5)*sizeof(int));while(tmp=dfs(S,INF)) Flow+=tmp;}return Flow;
}
int dis[22*22][22*22];
bool vis[22][22];
char map[22][22];
int num[22][22];
struct Node
{int x,y,d;Node(const int &a,const int &b,const int &c){x=a;y=b;d=c;}Node(){}
};
const int dx[]={0,1,0,-1},dy[]={1,0,-1,0};
int man[22*22],door[22*22],summ,sumd;
int main()
{scanf("%d%d",&N,&M);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){num[i][j]=++en;if(map[i][j]=='.') man[++summ]=en;else if(map[i][j]=='D') door[++sumd]=en;}memset(dis,0x7f,sizeof(dis));queue<Node>q;for(int i=1;i<=N;++i)for(int j=1;j<=M;++j)if(map[i][j]=='.'){memset(vis,0,sizeof(vis));q.push(Node(i,j,0));vis[i][j]=1;while(!q.empty()){Node U=q.front(); q.pop();for(int k=0;k<4;++k){int tx=U.x+dx[k],ty=U.y+dy[k];if(tx>=1 && tx<=N && ty>=1 && ty<=M && map[tx][ty]!='X' && (!vis[tx][ty])){if(map[tx][ty]=='D'){dis[num[i][j]][num[tx][ty]]=U.d+1;continue;//注意：到了门立刻离开，不能继续。 }vis[tx][ty]=1;q.push(Node(tx,ty,U.d+1));}}}}for(int Time=0;Time<=N*M;++Time){Init_Dinic();for(int i=1;i<=summ;++i) AddEdge(S,man[i],1);for(int i=1;i<=sumd;++i) AddEdge(door[i],T,Time);for(int i=1;i<=summ;++i)for(int j=1;j<=sumd;++j)if(dis[man[i]][door[j]]<=Time)AddEdge(man[i],door[j],1);if(max_flow()==summ){printf("%d\n",Time);return 0;}}puts("impossible");return 0;
}

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<queue>
using namespace std;
#define INF 2147483647
#define MAXN 411
#define MAXM 350001
int v[MAXM],cap[MAXM],en,first[MAXN],next[MAXM];
int d[MAXN],cur[MAXN];
queue<int>q;
int n,S,T,N,M;
void Init_Dinic(){memset(first,-1,sizeof(first)); en=0; S=0; n=N*M+2; T=N*M+1;}
void AddEdge(const int &U,const int &V,const int &W)
{v[en]=V; cap[en]=W; next[en]=first[U]; first[U]=en++;
v[en]=U; next[en]=first[V]; first[V]=en++;}
bool bfs()
{memset(d,-1,sizeof(d)); q.push(S); d[S]=0;while(!q.empty()){int U=q.front(); q.pop();for(int i=first[U];i!=-1;i=next[i])if(d[v[i]]==-1 && cap[i]){d[v[i]]=d[U]+1;q.push(v[i]);}}return d[T]!=-1;
}
int dfs(int U,int a)
{if(U==T || !a) return a;int Flow=0,f;for(int &i=cur[U];i!=-1;i=next[i])if(d[U]+1==d[v[i]] && (f=dfs(v[i],min(a,cap[i])))){cap[i]-=f; cap[i^1]+=f;Flow+=f; a-=f; if(!a) break;}if(!Flow) d[U]=-1;return Flow;
}
int max_flow()
{int Flow=0,tmp=0;while(bfs()){memcpy(cur,first,(n+5)*sizeof(int));while(tmp=dfs(S,INF)) Flow+=tmp;}return Flow;
}
int dis[22*22][22*22];
bool vis[22][22];
char map[22][22];
int num[22][22];
struct Node
{int x,y,d;Node(const int &a,const int &b,const int &c){x=a;y=b;d=c;}Node(){}
};
const int dx[]={0,1,0,-1},dy[]={1,0,-1,0};
int man[22*22],door[22*22],summ,sumd;
void Rebuild(const int &Time)
{Init_Dinic();for(int i=1;i<=summ;++i) AddEdge(S,man[i],1);for(int i=1;i<=sumd;++i) AddEdge(door[i],T,Time);for(int i=1;i<=summ;++i)for(int j=1;j<=sumd;++j)if(dis[man[i]][door[j]]<=Time)AddEdge(man[i],door[j],1);
}
int main()
{scanf("%d%d",&N,&M);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){num[i][j]=++en;if(map[i][j]=='.') man[++summ]=en;else if(map[i][j]=='D') door[++sumd]=en;}memset(dis,0x7f,sizeof(dis));queue<Node>q;for(int i=1;i<=N;++i)for(int j=1;j<=M;++j)if(map[i][j]=='.'){memset(vis,0,sizeof(vis));q.push(Node(i,j,0));vis[i][j]=1;while(!q.empty()){Node U=q.front(); q.pop();for(int k=0;k<4;++k){int tx=U.x+dx[k],ty=U.y+dy[k];if(tx>=1 && tx<=N && ty>=1 && ty<=M && map[tx][ty]!='X' && (!vis[tx][ty])){if(map[tx][ty]=='D'){dis[num[i][j]][num[tx][ty]]=U.d+1;continue;//注意：到了门立刻离开，不能继续。 }vis[tx][ty]=1;q.push(Node(tx,ty,U.d+1));}}}}int sz=sqrt(N*M),last=0;for(int Time=0;last<=N*M;Time+=sz){Rebuild(Time);if(max_flow()>=summ){for(int i=last+1;i<=Time;++i){Rebuild(i);if(max_flow()==summ){printf("%d\n",i); return 0;}}return 0;} last=Time;}puts("impossible");return 0;
}