//#include <iostream>
#include <queue>
#include <fstream>
#include <cstring>

using namespace std;

ifstream cin("barbar.in");
ofstream cout("barbar.out");
struct coord {
    int x, y, cost;
};

queue <coord> q;

int l,c,begi,begj,endi,endj,b[1005][1005];
char a[1005][1005],v[1005][1005];

void lee()
{
    coord w,w2;
    int t;
    int dx[]={-1,0,1,0};
    int dy[]={0,1,0,-1};
    while(q.empty()==false)
    {
        w=q.front();
        q.pop();
        for(t=0;t<4;t++)
        {
            w2.x=w.x+dx[t];
            w2.y=w.y+dy[t];
            w2.cost=w.cost+1;
            if(a[w2.x][w2.y]!='*')
                if(b[w2.x][w2.y]==0)
                {
                    q.push(w2);
                    b[w2.x][w2.y]=w2.cost;
                }
        }
    }
}

bool valid(int d)
{
    if(b[begi][begj]<=d)
        return false;
    memset(v,'0',sizeof(v));
    v[begi][begj]='1';
    while(!q.empty())
        q.pop();
    coord aux,w;
    int t;
    int dx[]={-1,0,1,0};
    int dy[]={0,1,0,-1};
    w.x=begi;w.y=begj;
    q.push(w);
    while(!q.empty())
    {
        w=q.front();
        q.pop();
        for(t=0;t<4;t++)
        {
            aux.x=w.x+dx[t];
            aux.y=w.y+dy[t];
            if(a[aux.x][aux.y]!='*' and b[aux.x][aux.y]>d and v[aux.x][aux.y]!='1')
            {
                if(aux.x==endi and aux.y==endj)
                    return true;
                v[aux.x][aux.y]='1';
                q.push(aux);
            }
        }

    }
    return false;

}

int main()
{
    int i,j,sol=-1,st,dr,d;
    coord aux;
    cin>>l>>c;
    for(i=1;i<=l;i++)
        cin>>a[i]+1;
    for(i=0;i<=c+1;i++)
        a[0][i]=a[l+1][i]='*';
    for(i=0;i<=l+1;i++)
        a[i][0]=a[i][c+1]='*';
    for(i=1;i<=l;i++)
        for(j=1;j<=c;j++)
            if(a[i][j]=='I')
            {
                begi=i;begj=j;
            }
            else
                if(a[i][j]=='O')
                {
                    endi=i;endj=j;
                }
                else
                    if(a[i][j]=='D')
                    {
                        aux.x=i;
                        aux.y=j;
                        aux.cost=1;
                        q.push(aux);
                        b[i][j]=1;
                    }
    lee();
    st=0;dr=1000;
    while(st<=dr)
    {
        d=(st+dr)>>1;
        if(valid(d))
        {
            sol=d;
            st=d+1;
        }
        else
            dr=d-1;
    }
    cout<<sol<<"\n";
    return 0;
}