/*
#include <fstream>
#include <queue>
#include <cstring>
using namespace std;

#define Nmax 1011
#define x first
#define y second

#define cost first
#define k1 second.first
#define k2 second.second

ifstream F("barbar.in");
ofstream G("barbar.out");

typedef pair<int,int> Pair;
typedef pair<int,Pair> Grupe;

queue< Pair > Q;

int D[Nmax][Nmax];
int Marked[Nmax][Nmax];
int Viz[Nmax][Nmax];
int Max;

int Lee[Nmax][Nmax];
queue< Pair > Q2;

char chr;
int N,M;

Pair Start,End;

int Dx[]={ 0 , -1 , 1 , 0 ,  0 };
int Dy[]={ 0 ,  0 , 0 , 1 , -1 };

void Get(int i,int j)
{
	F>>chr;
	switch( chr )
	{
		case 'I':
			Start.x=i;
			Start.y=i;
			break;
		case 'O':
			End.x=i;
			End.y=i;
			break;
		case '.':
			break;
		case 'D':
			D[i][j]=-1;
			Q.push( make_pair(i,j) );
			break;
		case '*':
			D[i][j]=-1;
			break;
	}
}

inline int in(int x,int y)
{
	if ( x>0 && x<=N && y>0 && y<=M ) 
		return 1;
	return 0;
}


bool Ver()
{
	Q2.push( Start );
	while ( Q2.size() )
	{
		int i=Q2.front().x;
		int j=Q2.front().y;
		
		for (int k=1;k<5;++k)
			if ( in(i+Dx[k], j+Dy[k]) )
				if ( !Lee[ i+Dx[k] ][ j+Dy[k] ] && !D[ i+Dx[k] ][ j+Dy[k] ] )
				{
					Lee[ i+Dx[k] ][ j+Dy[k] ]=1;
					Q2.push( make_pair(i+Dx[k],j+Dy[k]) );
				}
		
		Q2.pop();
	}
	return Lee[End.x][End.y];
}

int Okay(int);

void Bs()
{
	int St,Dr,Mij;
	
	St=0;
	Dr=N*M;
	
	while ( St<=Dr )
	{
		Mij=(St+Dr)/2;
		
		if( Okay(Mij) )		
		{
			Max=max(Max,Mij);
			St=Mij+1;
		}
		else
			Dr=Mij-1;
	}
}

int main()
{
	F>>N>>M;
	
	for (int i=1;i<=N;++i)
		for (int j=1;j<=N;++j)
			Get(i,j);
	
	if ( ! Ver() )
	{
		G<<"-1\n";
		return 0;
	}		
	
	while ( Q.size() )
	{
		int i=Q.front().x;
		int j=Q.front().y;
		
		for (int k=1;k<5;++k)
			if ( in(i+Dx[k], j+Dy[k]) )
				if ( !D[ i+Dx[k] ][ j+Dy[k] ] )
				{
					D[ i+Dx[k] ][ j+Dy[k] ]=(D[i][j]==-1 ? 0 : D[i][j])+1;
					Q.push( make_pair(i+Dx[k],j+Dy[k]) );
				}
		
		Q.pop();
	}
	
	Bs();
	
	G<<Max<<'\n';
}

int Okay( int Val )
{
	int i,j,k;
	for( i=1;i<=N;i++)
		for( j=1;j<=M;j++)
			Viz[i][j]=0;
	while( !Q.empty() ) Q.pop();
	
	Pair aux,poz;
	Q.push( Start );
	
	Viz[Start.x][Start.y]=1;
	
	if ( D[Start.x][Start.y] < Val)
		return 0;
	
	while( !Q.empty() )
	{
		aux = Q.front();
		Q.pop();
		
		if( aux.x==End.x && aux.y==End.y ) return 1;
		
		for( k=1;k<5;k++ )
		{
			poz.x=aux.x+Dx[k];
			poz.y=aux.y+Dy[k];
			
			if(!Viz[poz.x][poz.y] && D[poz.x][poz.y]!=-1 && D[poz.x][poz.y]>=Val && in(poz.x,poz.y))
			{
				Viz[poz.x][poz.y]=1;
				Q.push( poz );
			}
		}
	}
	return 0;
}
*/
#include <stdio.h>
#include <string.h>
#include <queue>

#define MAX 1011

using namespace std;

int matrice[MAX][MAX], dist[MAX][MAX], r, c, sol = -1;
int dX[4] = {-1, 0, 1, 0}, dY[4] = {0, 1, 0, -1};
bool visit[MAX][MAX];

struct point {int x, y; }start, stop;

queue<point> q;

void citire()
{
    freopen("barbar.in", "r", stdin);
    scanf("%d %d\n", &r, &c);
    int i, j;
    char aux;
    point add;
    for(i = 1; i <= r; i++)
    {
        for(j = 1; j <= c; j++)
        {
            scanf("%c", &aux);
            if(aux == 'D')
            {
                dist[i][j] = 1;
                visit[i][j] = true;
                add.x = i;
                add.y = j;
                q.push(add);
            }
            else if(aux == 'I')
            {
                start.x = i;
                start.y = j;
            }
            else if(aux == 'O')
            {
                stop.x = i;
                stop.y = j;
            }
            else if(aux == '*')
            {
                visit[i][j] = true;
                dist[i][j] = -1;
            }
        }
        scanf("\n");
    }
    fclose(stdin);
    for(i = 0; i <= c + 1; i++)
        dist[0][i] = dist[r + 1][i] = -1;
    for(i = 0; i <= r + 1; i++)
        dist[i][0] = dist[i][c + 1] = -1;
}

void genDist()
{
    int x, y, X, Y, i;
    point add;
    while(!q.empty())
    {
        x = q.front().x; y = q.front().y;
        q.pop();
        for(i = 0; i < 4; i++)
        {
            X = x + dX[i];
            Y = y + dY[i];
            if(!dist[X][Y])
            {
                dist[X][Y] = dist[x][y] + 1;
                visit[X][Y] = true;
                add.x = X; add.y = Y; q.push(add);
            }
            else
                if(dist[X][Y] > dist[x][y] + 1)
                    dist[X][Y] = dist[x][y] + 1;
        }
    }
}

void prepare()
{
    memset(visit, 0, MAX * MAX * sizeof(bool));
    while(!q.empty())
        q.pop();
    point add;
    add.x = start.x;
    add.y = start.y;
    q.push(add);
    visit[start.x][start.y] = true;
}

bool lee(int minimum)
{
    int x, y, X, Y, i;
    point add;
    while(!q.empty())
    {
        x = q.front().x; y = q.front().y;
        q.pop();
        for(i = 0; i < 4; i++)
        {
            X = x + dX[i];
            Y = y + dY[i];
            if(dist[X][Y] >= minimum && matrice[X][Y] >= 0 && !visit[X][Y])
            {
                visit[X][Y] = true;
                add.x = X; add.y = Y; q.push(add);
            }
            if(X == stop.x && Y == stop.y)
                return true;
        }
    }
    return false;
}

void cautare()
{
    int begin = 1, end = min(dist[start.x][start.y], dist[stop.x][stop.y]), mid = (begin + end) / 2;
    while(begin <= end)
    {
        prepare();
        if(lee(mid))
        {
            begin = mid + 1;
            sol = mid - 1;
        }
        else
            end = mid - 1;
        mid = (begin + end) / 2;
    }
}

void afisare()
{
    freopen("barbar.out", "w", stdout);
    printf("%d", sol);
    fclose(stdout);
}

int main()
{
    citire();
    genDist();
    cautare();
    afisare();
    return 0;
}