#include <iostream>
#include <algorithm>
#include<cmath>
#include<cstring>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<iomanip>
#include<ctime>
#include<set>
#include<map>
#include<queue>
#include<stack>
#define sqr(x) ((x)*(x))
#define fz1(i,n) for ((i)=1;(i)<=(n);(i)++)
#define fd1(i,n) for ((i)=(n);(i)>=1;(i)--)
#define fz0g(i,n) for ((i)=0;(i)<=(n);(i)++)
#define fd0g(i,n) for ((i)=(n);(i)>=0;(i)--)
#define fz0k(i,n) for ((i)=0;(i)<(n);(i)++)
#define fd0k(i,n) for ((i)=(long long)((n)-1);(i)>=0;(i)--)
#define fz(i,x,y) for ((i)=(x);(i)<=(y);(i)++)
#define fd(i,y,x) for ((i)=(y);(i)>=(x);(i)--)
#define fzin fz1(i,n)
#define fzim fz1(i,m)
#define fzjn fz1(j,n)
#define fzjm fz1(j,m)
#define ff(c,itr) for (__typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)
#define rdst(st,len){static char ss[len];scanf(" %s",ss);(st)=ss;}
//#define T_ int
using namespace std;
//struct bit{T_ arr[1000005];int lowbit(int x){return x&(-x);}void add(int x,T_ y){for (int i=x;i<=n;i+=lowbit(i)) arr[i]+=y;}T_ query(int x){T_ s=0;for (int i=x;i>0;i-=lowbit(i)) s+=arr[i];return s;}};
//struct seg{struct treedot{int l,r;T_ sum,add;void update(T_ x){sum+=(r-l+1)*x; add+=x;}}c[800005];void pushdown(int id){if (c[id].add){c[id*2].update(c[id].add);c[id*2+1].update(c[id].add);c[id].add=0;}}void pushup(int id){c[id].sum=c[id*2].sum+c[id*2+1].sum;}void build(int id,int l,int r){if (l>r) return;c[id].l=l;c[id].r=r;c[id].sum=0;c[id].add=0;if (l==r) c[id].sum=0;else{int mid=l+(r-l)/2;build(id*2,l,mid);build(id*2+1,mid+1,r);pushup(id);}}void update(int id,int le,int ri,T_ x){if (le>c[id].r||ri<c[id].l) return;if (le<=c[id].l&&c[id].r<=ri) c[id].update(x);else{pushdown(id);update(id*2,le,ri,x);update(id*2+1,le,ri,x);pushup(id);}}T_ query(int id,int le,int ri){if (ri<c[id].l||c[id].r<le) return 0;if (le<=c[id].l&&c[id].r<=ri) return c[id].sum;T_ ans=0;pushdown(id);ans+=query(id*2,le,ri);ans+=query(id*2+1,le,ri);pushup(id);return ans;}};
//struct bit_2d{T_ a[2005][2005];int lowbit(int x){return x&(-x);}void add(int x,int y,T_ z){int i,j;for (i=x;i<=2000;i+=lowbit(i)){for (j=y;j<=2000;j+=lowbit(j)){a[i][j]+=z;}}}T_ query(int x,int y){int i,j;T_ s=0;for (i=x;i;i-=lowbit(i)){for (j=y;j;j-=lowbit(j)){s+=a[i][j];}}return s;}T_ query(int x,int y,int xx,int yy){return query(xx,yy)-query(x-1,yy)-query(xx,y-1)+query(x-1,y-1);}};
int n,m,i,j,k,p,f[195][195][195],g[195][195][195],nf[195][195][195],ng[195][195][195],l,r,mid,ans;
char c;int a[195][195];
int query(int x,int y,int xx,int yy)
{
	return a[xx][yy]-a[xx][y-1]-a[x-1][yy]+a[x-1][y-1];
}
bool check(int x,int y,int xx,int yy)
{
	int t=query(x,y,xx,yy);
	if (t==0||t==(xx-x+1)*(yy-y+1)) return 1;
	return 0;
}
int main()
{
	scanf("%d%d",&n,&m);
	fz1(i,n){
		scanf(" ");
		fz1(j,m){
			scanf("%c",&c);
			if (c=='#') a[i][j]=1;
			a[i][j]+=a[i-1][j]+a[i][j-1]-a[i-1][j-1];
		}
	}
	fz1(i,n) fz1(j,m) fz(k,i,n){
		ans=j-1;l=j;r=m+1;
		while (l<r){
			mid=(l+r)/2;
			if (check(i,j,k,mid)){
				ans=mid;l=mid+1;
			}
			else r=mid;
		}
		f[i][j][k]=ans;
	}
	fz1(i,n) fz1(j,m) fz(k,j,m){
		ans=i-1;l=i;r=n+1;
		while (l<r){
			mid=(l+r)/2;
			if (check(i,j,mid,k)){
				ans=mid;l=mid+1;
			}
			else r=mid;
		}
		g[i][j][k]=ans;
	}
	for (ans=0;;ans++){
		if (f[1][1][n]==m||g[1][1][m]==n){
			cout<<ans<<endl;
			return 0;
		}
		fz1(i,n) fz1(j,m) fz(k,i,n){
			int t=f[i][j][k];
			if (t<m) nf[i][j][k]=f[i][t+1][k]; else nf[i][j][k]=t;
		}
		fz1(i,n) fz1(j,m) fz(k,j,m){
			int t=g[i][j][k];
			if (t<n) ng[i][j][k]=g[t+1][j][k]; else ng[i][j][k]=t;
		}
		fz1(i,n) fz1(j,m) fz(k,i,n){
			int t=nf[i][j][k];
			while (t<m&&ng[i][j][t+1]>=k) t++;
			f[i][j][k]=t;
		}
		fz1(i,n) fz1(j,m) fz(k,j,m){
			int t=ng[i][j][k];
			while (t<n&&nf[i][j][t+1]>=k) t++;
			g[i][j][k]=t;
		}
	}
	return 0;
}

Measured memory_bytes: 114816