#include <cstdio>
#include <cstring>
#include <algorithm>
#define fo(i,a,b) for(int i=a;i<=b;i++)
#define fd(i,a,b) for(int i=a;i>=b;i--)
using namespace std;

const int N=225;

int f[N][N][N],g[N][N][N],n,m;
char s[N][N];

int main() {
	scanf("%d%d",&n,&m);
	fo(i,1,n) scanf("%s",s[i]+1);
	fo(i,1,n) {
		f[i][i][m]=m;
		fd(j,m-1,1) {
			if (s[i][j]==s[i][j+1]) f[i][i][j]=f[i][i][j+1];
			else f[i][i][j]=j;
		}
	}
	fo(i,1,n) 
		fo(k,1,m) 
			fo(j,i+1,n) {
				if (s[j][k]!=s[j-1][k]) break;
				f[i][j][k]=min(f[i][j-1][k],f[j][j][k]);
			}
	int ans=0;
	while (f[1][n][1]<m) {
		fo(i,1,n) fo(j,i,n) fo(k,1,m) g[i][j][k]=f[i][j][k]<m?f[i][j][f[i][j][k]+1]:m;
		fo(i,1,n)
			fo(k,1,m) {
				int p=i;
				fo(j,i,n) {
					while (p<j&&min(f[i][p][k],f[p+1][j][k])<=min(f[i][p+1][k],f[p+2][j][k])) p++;
					g[i][j][k]=max(g[i][j][k],min(f[i][p][k],f[p+1][j][k]));
				}
			}
		fo(i,1,n) fo(j,i,n) fo(k,1,m) f[i][j][k]=g[i][j][k];
		ans++;
	}
	printf("%d\n",ans);
	return 0;
}