#include <bits/stdc++.h>
using namespace std;
int harta[1005][1005],i,j,R,C,par[1005][1005],L1,C1,L2,C2,maxim=-1,st,dr,mij,bun=-1,x;
char c,a[1005][1005];
ifstream f("barbar.in");
ofstream g("barbar.out");
queue<pair<int,int>> q;
void creeare(){
    int d,dx[]={-1,0,1,0},dy[]={0,1,0,-1},i1,j1,inou,jnou;
    while(!q.empty()){
        i1=q.front().first;
        j1=q.front().second;
        q.pop();
        for(d=0;d<4;d++){
            inou=i1+dx[d];
            jnou=j1+dy[d];
            if(inou>=1 && inou<=R && jnou>=1 && jnou<=C && a[inou][jnou]!='*' && harta[inou][jnou]==-1){
                harta[inou][jnou]=harta[i1][j1]+1;
                maxim=max(harta[inou][jnou],maxim);
                q.push({inou,jnou});
            }
        }
    }
}
bool parcurgere(int starti,int startj,int final1,int final2,int verif){
    int d,dx[]={-1,0,1,0},dy[]={0,1,0,-1},i1,j1,inou,jnou;
    queue<pair<int,int>> q1;
    q1.push({starti,startj});
    if(harta[starti][startj]<verif){
        return false;
    }
    par[starti][startj]=0;
    while(!q1.empty()){
        i1=q1.front().first;
        j1=q1.front().second;
        q1.pop();
        for(d=0;d<4;d++){
        inou=i1+dx[d];
        jnou=j1+dy[d];
        if(inou>=1 && inou<=R && jnou>=1 && jnou<=C && harta[inou][jnou]>=verif && harta[inou][jnou]!=-1){
                if(inou==L2 && jnou==C2){
                    return true;
                }
                if(par[inou][jnou]==-1){
                par[inou][jnou]=par[i1][j1]+1;
                q1.push({inou,jnou});
                }
        }
        }
    }
    return false;
}
int main()
{
    memset(harta,-1,sizeof(harta));
    memset(par,-1,sizeof(par));
    f>>R>>C;
    for(i=1;i<=R;i++){
        for(j=1;j<=C;j++){
            f>>c;
            a[i][j]=c;
        }
    }
    for(i=1;i<=R;i++){
        for(j=1;j<=C;j++){
            if(a[i][j]=='D'){
                harta[i][j]=0;
                q.push({i,j});
            }
            if(a[i][j]=='I'){
                L1=i;
                C1=j;
            }
            if(a[i][j]=='O'){
                L2=i;
                C2=j;
            }            
        }
    }
    creeare();
    st=0;
    dr=maxim;
    while(st<=dr){
        x=(st+dr)/2;
        if(parcurgere(L1,C1,L2,C2,x)){
            bun=max(bun,x);
            st=x+1;
            memset(par,-1,sizeof(par));
        }
        else{
            dr=x-1;
        }
    }
    g<<bun;
    return 0;
}