#include <bits/stdc++.h>
#include <fstream>
#include <queue>

using namespace std;
ifstream fin("barbar.in");
ofstream fout("barbar.out");

int const INF=INT_MAX/2;
int const di[]={-1,0,1,0};
int const dj[]={0,1,0,-1};
int dist[1001][1001],drag[1001][1001];
char viz[1001][1001];
queue<pair<int,int> >q;
int n,m,xx,yy,xf,yf;
bool pos(int a,int b)
{
    return a<=n && a>0 && b>0 && b<=m;
}
void bfs()
{
    while(!q.empty())
    {
        int xi=q.front().first;
        int yi=q.front().second;
        q.pop();
        for(int k=0;k<4;++k)
        {
            int a=xi+di[k];
            int b=yi+dj[k];
            if(drag[a][b]>drag[xi][yi]+1 && viz[a][b]!='*' && pos(a, b))
            {
                q.push({a,b});
                drag[a][b]=drag[xi][yi]+1;
            }
        }
    }
}
bool verif(int mij,int xx,int yy,int xf,int yf)
{
    if(drag[xx][yy] < mij)
        return 0;
    for(int i=1;i<=n;++i)
        for(int j=1;j<=m;++j)
          dist[i][j]=INF;
    q.push({xx,yy});
    dist[xx][yy]=0;
    while(!q.empty())
    {
        int xi=q.front().first;
        int yi=q.front().second;
        q.pop();
        for(int k=0;k<4;++k)
        {
            int a=xi+di[k];
            int b=yi+dj[k];
            if(pos(a,b) && viz[a][b]!='*' && drag[a][b]>=mij && dist[xi][yi]+1<dist[a][b])
            {
                q.push({a,b});
                dist[a][b]=dist[xi][yi]+1;
            }
        }
    }
    return (dist[xf][yf] != INF);
}
void cautare_binara(int st,int dr)
{
    while(st<dr)
    {
         int mij=(st+dr + 1)/2;
         if(verif(mij,xx,yy,xf,yf)) st = mij;
            else dr=mij - 1;
    }
    if(st == 0)
    {
        if(verif(st,xx,yy,xf,yf))fout << st;
        else fout << -1;
    }
    else fout << st;
}
int main()
{
    fin>>n>>m;
    for(int i=1;i<=n;++i)
        for(int j=1;j<=m;++j)
          drag[i][j]=INF;
    int i,j;
    char p;
    for(i=1;i<=n;++i)
        for(j=1;j<=m;++j)
    {
        fin>>viz[i][j];
        char p=viz[i][j];
            if(p=='I')
            {
                xx=i;yy=j;
            }
        else
            if(p=='D')
            {
                q.push({i,j});
                drag[i][j]=0;
            }
        else
            if(p=='O')
            {
                xf=i;yf=j;
            }
    }
    bfs();
    cautare_binara(0,max(n,m)+1);
    return 0;
}