#include <bits/stdc++.h>

//    #include <boost/multiprecision/cpp_int.hpp>
 #define int long long
 #define inf  1000000007
 #define pa pair<int,int>
 #define ll long long
 #define pal pair<double,double>
 #define ppap pair<pa,int>
  #define PI 3.14159265358979323846
  #define paa pair<int,char>
  #define  mp make_pair
  #define  pb push_back
  #define EPS (1e-10)
                                          
    int dx[8]={0,1,0,-1,1,1,-1,-1};
    int dy[8]={1,0,-1,0,-1,1,1,-1};
                                            using namespace std;
                                   			class pa3{
                                            	public:
                                            	int x;
                                   				int y,z;
                                            	pa3(int x=0,int y=0,int z=0):x(x),y(y),z(z) {}
                                            	bool operator < (const pa3 &p) const{
                                            		if(x!=p.x) return x<p.x;
                                            		if(y!=p.y) return y<p.y;
                                            		 return z<p.z;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                   				bool operator > (const pa3 &p) const{
                                            		if(x!=p.x) return x>p.x;
                                            		if(y!=p.y) return y>p.y;
                                            		 return z>p.z;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa3 &p) const{
                                            		return x==p.x && y==p.y && z==p.z;
                                            	}
                                            		bool operator != (const pa3 &p) const{
                                            			return !( x==p.x && y==p.y && z==p.z);
                                            	}
                                            
                                            };
                                            
                                            class pa4{
                                            	public:
                                            	int x;
                                            	int y,z,w;
                                            	pa4(int x=0,int y=0,int z=0,int w=0):x(x),y(y),z(z),w(w) {}
                                            	bool operator < (const pa4 &p) const{
                                            		if(x!=p.x) return x<p.x;
                                            		if(y!=p.y) return y<p.y;
                                            		if(z!=p.z)return z<p.z;
                                            		return w<p.w;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator > (const pa4 &p) const{
                                            		if(x!=p.x) return x>p.x;
                                            		if(y!=p.y) return y>p.y;
                                            		if(z!=p.z)return z>p.z;
                                            		return w>p.w;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa4 &p) const{
                                            		return x==p.x && y==p.y && z==p.z &&w==p.w;
                                            	}
                                            		
                                            
                                            };
                                            class pa2{
                                            	public:
                                            	int x,y;
                                            	pa2(int x=0,int y=0):x(x),y(y) {}
                                            	pa2 operator + (pa2 p) {return pa2(x+p.x,y+p.y);}
                                            	pa2 operator - (pa2 p) {return pa2(x-p.x,y-p.y);}
                                            	bool operator < (const pa2 &p) const{
                                            		return y != p.y ? y<p.y: x<p.x;
                                            	}
                                            	bool operator > (const pa2 &p) const{
                                            		return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa2 &p) const{
                                            		return abs(x-p.x)==0 && abs(y-p.y)==0;
                                            	}
                                            	bool operator != (const pa2 &p) const{
                                            		return !(abs(x-p.x)==0 && abs(y-p.y)==0);
                                            	}
                                            		
                                            
                                            };
                                            

                      
                                string itos( int i ) {
                                ostringstream s ;
                                s << i ;
                                return s.str() ;
                                }
                                 
                                int gcd(int v,int b){
                                	if(v>b) return gcd(b,v);
                                	if(v==b) return b;
                                	if(b%v==0) return v;
                                	return gcd(v,b%v);
                                }
                 
                            
                                int mod;
int extgcd(int a, int b, int &x, int &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    int d = extgcd(b, a%b, y, x);
    y -= a/b * x;
    return d;
}
pa operator+(const pa & l,const pa & r) {   
    return {l.first+r.first,l.second+r.second};                                    
}    
pa operator-(const pa & l,const pa & r) {   
    return {l.first-r.first,l.second-r.second};                                    
}  
                                
                int pr[1200010];
                int inv[1200010];
                
                int beki(int wa,int rr,int warukazu){
                	if(rr==0) return 1%warukazu;
                	if(rr==1) return wa%warukazu;
                	wa%=warukazu;
                	if(rr%2==1) return ((ll)beki(wa,rr-1,warukazu)*(ll)wa)%warukazu;
                	ll zx=beki(wa,rr/2,warukazu);
                	return (zx*zx)%warukazu;
                }

                
    			int comb(int nn,int rr){
    				if(rr<0 || rr>nn || nn<0) return 0;
    				int r=pr[nn]*inv[rr];
    				r%=mod;
    				r*=inv[nn-rr];
    				r%=mod;
    				return r;
    			}
                
                void gya(int ert){
                	pr[0]=1;
                	for(int i=1;i<=ert;i++){
                		pr[i]=(pr[i-1]*i)%mod;
                	}
                		inv[ert]=beki(pr[ert],mod-2,mod);
                	for(int i=ert-1;i>=0;i--){
                		inv[i]=inv[i+1]*(i+1)%mod;
                	}
                }
                
              //   cin.tie(0);
    		//	ios::sync_with_stdio(false);
    			//priority_queue<pa3,vector<pa3>,greater<pa3>> pq;            
                 //sort(ve.begin(),ve.end(),greater<int>());
//    mt19937(clock_per_sec);
  //  mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count()) ;
      
       
       
                               //----------------kokomade tenpure------------
int dp1[2][200][200][200];

int dp2[2][200][200][200];


int sita[200][200];
int migi[200][200];

int a[200][200]={};
 signed main(){
 	

    	       cin.tie(0);
   			ios::sync_with_stdio(false);
int h,w;
 	cin>>h>>w;
 	
 	for(int i=0;i<h;i++){
 		string s;
 		cin>>s;
 		
 		for(int j=0;j<w;j++){
 			if(s[j]=='.')a[i][j]=1;
 			else a[i][j]=2;
 		}
 	}
 	
 	for(int i=h-1;i>=0;i--)for(int j=w-1;j>=0;j--){
 		if(a[i+1][j]!=a[i][j]){
 			sita[i][j]=i+1;
 		}
 		else sita[i][j]=sita[i+1][j];
 		
 		if(a[i][j+1]!=a[i][j]){
 			migi[i][j]=j+1;
 		}
 		else migi[i][j]=migi[i][j+1];
 	}
 	
 	for(int i=0;i<=h;i++)migi[i][w]=w,sita[i][w]=h;
 	for(int i=0;i<=w;i++)sita[h][i]=h,migi[h][i]=w;
 	
 	for(int x1=0;x1<=h;x1++)for(int y1=0;y1<=w;y1++){
 		for(int x2=x1;x2<=h;x2++){
 			if(x1==x2){
 				dp1[0][x1][x2][y1]=w;
 			}
 			else if(x2==x1+1){
 				dp1[0][x1][x2][y1]=migi[x1][y1];
 			}
 			else{
 				if(dp1[0][x1][x2-1][y1]==y1)dp1[0][x1][x2][y1]=y1;
 				else if(a[x2-1][y1]!=a[x2-2][y1]){
 					dp1[0][x1][x2][y1]=y1;
 				}
 				else{
 					dp1[0][x1][x2][y1]=min(dp1[0][x1][x2-1][y1],migi[x2-1][y1]);
 				}
 			}
 			
 	//		cout<<"s "<<x1<<" "<<x2<<" "<<y1<<" "<<dp1[0][x1][x2][y1]<<endl;
 		}
 		
 //		cout<<endl;
 		for(int y2=y1;y2<=w;y2++){
 			if(y1==y2){
 				dp2[0][x1][y1][y2]=h;
 			}
 			else if(y2==y1+1){
 				dp2[0][x1][y1][y2]=sita[x1][y1];
 			}
 			else{
 				if(dp2[0][x1][y1][y2-1]==x1)dp2[0][x1][y1][y2]=x1;
 				else if(a[x1][y2-1]!=a[x1][y2-2]){
 					dp2[0][x1][y1][y2]=x1;
 				}
 				else{
 					dp2[0][x1][y1][y2]=min(dp2[0][x1][y1][y2-1],sita[x1][y2-1]);
 				}
 			}
 	//		cout<<"t "<<x1<<" "<<y1<<" "<<y2<<" "<<dp2[0][x1][y1][y2]<<endl;
 		}
 	}
 //	cout<<endl;
 	
 	for(int cnt=1;cnt<500;cnt++){
 	//	cout<<"==="<<cnt<<"==="<<endl;
 		int b=(cnt&1);
 	//	cout<<cnt<<" "<<dp1[b^1][0][h][0]<<endl;
 		if(dp1[b^1][0][h][0]==w){
 			cout<<cnt-1<<endl;
 			return 0;
 		}
 		
 		
 		for(int x1=0;x1<=h;x1++)for(int y1=0;y1<=w;y1++){
 			for(int x2=x1;x2<=h;x2++){
 				dp1[b][x1][x2][y1]=dp1[b^1][x1][x2][dp1[b^1][x1][x2][y1]];
 			}
 			for(int y2=y1;y2<=w;y2++){
 				dp2[b][x1][y1][y2]=dp2[b^1][dp2[b^1][x1][y1][y2]][y1][y2];
 			}
 			
 			int s1=w;
 			for(int i=x1;i<=h;i++){
 				while(dp2[b^1][dp2[b^1][x1][y1][s1]][y1][s1]<i)s1--;
 				
 				
 				dp1[b][x1][i][y1]=max(dp1[b][x1][i][y1],s1);
 				
 //				cout<<"dp1 "<<x1<<" "<<i<<" "<<y1<<" "<<dp1[b][x1][i][y1]<<endl;
 			}
 			
 			s1=h;
 			
 			for(int i=y1;i<=w;i++){
 				while(dp1[b^1][x1][s1][dp1[b^1][x1][s1][y1]]<i)s1--;
 	//			while(dp1[b^1][x1][s1][y1]<i)s1--;
 	//			while(dp1[b^1][s1][s2][y1]<i)s2--;
 				
 				dp2[b][x1][y1][i]=max(dp2[b][x1][y1][i],s1);
 	//			cout<<"dp2 "<<x1<<" "<<y1<<" "<<i<<" "<<dp2[b][x1][y1][i]<<endl;
 			}
 			
 			
 		}
 		
 		
 		
 	}
 	
 	return 0;

  }
  

Measured memory_bytes: 239744