#include <bits/stdc++.h>
const int N=190;
using std::max;
using std::min;
int n,m,a[N][N],sum[N][N],dp[N][N][N][15];
char c[N][N];
int main(){
    scanf("%d%d",&n,&m);
    for (int i=1;i<=n;i++) scanf("%s",c[i]+1);
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++){
            if (c[i][j]=='.') a[i][j]=1;
            else a[i][j]=0;
            sum[i][j]=sum[i-1][j]+sum[i][j-1]-sum[i-1][j-1]+a[i][j];
        }
    for (int i=1;i<=n;i++)  
        for (int j=1;j<=m;j++)
            for (int k=j;k<=m;k++){
                int l=i,r=n,ans=i-1;
                while (l<=r){
                    int mid=(l+r)>>1;
                    int s=sum[mid][k]-sum[mid][j-1]-sum[i-1][k]+sum[i-1][j-1];
                    if (s==0 || s==(mid-i+1)*(k-j+1))
                        ans=mid,l=mid+1;
                    else r=mid-1;
                }
                dp[i][j][k][0]=ans;
            }
    int k=1;
    for (k=1;dp[1][1][m][k-1]<n;k++)
    for (int h=1;h<=m;h++)
        for (int i=1;i<=n;i++)
            for (int j1=1,j2=j1+h-1;j2<=m;j1++,j2++){
                    if (dp[i][j1][j2][k-1]==n){
                        dp[i][j1][j2][k]=n;
                        continue;
                    }
                    dp[i][j1][j2][k]=dp[dp[i][j1][j2][k-1]+1][j1][j2][k-1];//横
                    if(dp[i][j1][j2][k]==n) continue; 
                    int l=j1,r=j2-1,ans=0;//竖 
                    while (l<=r){
                        int mid=(l+r)>>1;
                        int x=dp[i][j1][mid][k-1],y=dp[i][mid+1][j2][k-1];
                        ans=max(ans,min(x,y));
                        if (x<y) r=mid-1;
                        else l=mid+1;
                    }
    
                    dp[i][j1][j2][k]=max(dp[i][j1][j2][k],ans);
                }
    printf("%d\n",k-1);
    return 0;
}