#include<bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define uint unsigned
#define pii pair<int,int>
#define pll pair<ll,ll>
#define PB push_back
#define fi first
#define se second
#define For(i,j,k) for (int i=(int)(j);i<=(int)(k);i++)
#define Rep(i,j,k) for (int i=(int)(j);i>=(int)(k);i--)
#define CLR(a,v) memset(a,v,sizeof(a));
#define CPY(a,b) memcpy(a,b,sizeof(a));
using namespace std;
const int N=205;
int n,m,fl[N];
char s[N][N];
int f[2][N][N][N];
int g[2][N][N][N];
int main(){
	scanf("%d%d",&n,&m);
	For(i,1,n) scanf("%s",s[i]+1);
	For(i,1,n) For(j,1,m)
		s[i][j]=(s[i][j]=='.'?0:1);
	For(i,1,n){
		For(j,1,m) fl[j]=s[i][j];
		For(j,i,n){
			For(k,1,m)
				if (s[j][k]!=fl[k])
					fl[k]=2;
			int las=fl[m],ed=m;
			f[0][i][j][m+1]=m+1;
			Rep(k,m,1){
				if (las!=fl[k]) las=fl[k],ed=k;
				f[0][i][j][k]=(las==2?k:ed+1);
			}
		}
	}
	For(i,1,m){
		For(j,1,n) fl[j]=s[j][i];
		For(j,i,m){
			For(k,1,n)
				if (s[k][j]!=fl[k])
					fl[k]=2;
			int las=fl[n],ed=n;
			g[0][i][j][n+1]=n+1;
			Rep(k,n,1){
				if (las!=fl[k]) las=fl[k],ed=k;
				g[0][i][j][k]=(las==2?k:ed+1);
			}
		}
	}
	int pr=0,su=1;
	For(ans,0,100){
		if (f[pr][1][n][1]==m+1){
			printf("%d\n",ans);
			return 0;
		}
		For(i,1,n) For(j,i,n) For(k,1,m+1)
			f[su][i][j][k]=f[pr][i][j][f[pr][i][j][k]];
		For(i,1,m) For(j,i,m) For(k,1,n+1)
			g[su][i][j][k]=g[pr][i][j][g[pr][i][j][k]];
		For(i,1,n) For(j,i,n){
			int ed=1;
			For(k,1,m){
				ed=max(ed,k);
				for (;ed<m+1&&g[pr][k][ed][g[pr][k][ed][i]]>j;++ed);
				f[su][i][j][k]=max(f[su][i][j][k],ed);
			}
		}
		For(i,1,m) For(j,i,m){
			int ed=1;
			For(k,1,n){
				ed=max(ed,k);
				for (;ed<n+1&&f[pr][k][ed][f[pr][k][ed][i]]>j;++ed);
				g[su][i][j][k]=max(g[su][i][j][k],ed);
			}
		}
		swap(pr,su);
	}
}