#include<bits/stdc++.h>
#define cs const
#define pb push_back
using namespace std;
cs int N = 200;
int n, m, f[N][N][N], g[N][N][N];
char mp[N][N];
int main(){
	#ifdef FSYolanda
	freopen("1.in","r",stdin);
	#endif
	scanf("%d%d",&n,&m);
	for(int i=1; i<=n; i++)
	scanf("%s",mp[i]+1);
	for(int l=1; l<=n; l++)
	for(int r=l; r<=n; r++){
		f[l][r][m+1]=m;
		for(int i=m; i>=1; i--){
			bool ok = l==r || (mp[l][i]==mp[r][i]&&f[l][r-1][i]>=i);
			if(!ok) f[l][r][i]=i-1;
			else f[l][r][i]=mp[l][i]==mp[l][i+1]?f[l][r][i+1]:i;
		}
	}
	for(int l=1; l<=m; l++)
	for(int r=l; r<=m; r++){
		g[l][r][n+1]=n;
		for(int i=n; i>=1; i--){
			bool ok = l==r || (mp[i][l]==mp[i][r]&&g[l][r-1][i]>=i);
			if(!ok) g[l][r][i]=i-1;
			else g[l][r][i]=mp[i][l]==mp[i+1][l]?g[l][r][i+1]:i;
		}
	} int tim = 0;
	while(true){
		if(f[1][n][1]>=m) return cout<<tim,0; ++tim;
		for(int l=1; l<=n; l++)
		for(int r=l; r<=n; r++)
		for(int i=1; i<=m; i++)
		f[l][r][i]=f[l][r][f[l][r][i]+1];
		for(int l=1; l<=m; l++)
		for(int r=l; r<=m; r++)
		for(int i=1; i<=n; i++)
		g[l][r][i]=g[l][r][g[l][r][i]+1];
		for(int i=1; i<=n; i++)
		for(int j=1; j<=m; j++){
			static int a[N], b[N];
			for(int k=i; k<=n; k++) a[k]=f[i][k][j];
			for(int k=j; k<=m; k++) b[k]=g[j][k][i];
			a[n+1]=j-1; b[m+1]=i-1;
			for(int k=i; k<=n; k++)
			for(int l=a[k];l>a[k+1];l--) 
			g[j][l][i]=max(g[j][l][i],k);
			for(int k=j; k<=m; k++)
			for(int l=b[k];l>b[k+1];l--)
			f[i][l][j]=max(f[i][l][j],k);
		}
	} return 0;
}