#include<cstdio>
#include<iostream>
#define RI register int
#define CI const int&
using namespace std;
const int N=200;
int n,m,f[N][N][N],g[N][N][N]; char a[N][N];
int main()
{
	RI i,j,k,p,t; for (scanf("%d%d",&n,&m),i=1;i<=n;++i) scanf("%s",a[i]+1);
	for (i=1;i<=n;++i) for (j=i;j<=n;++j)
	for (f[i][j][m+1]=k=m;k;--k) if (i==j||(a[i][k]==a[j][k]&&f[i][j-1][k]>=k))
	f[i][j][k]=a[i][k]==a[i][k+1]?f[i][j][k+1]:k; else f[i][j][k]=k-1;
	for (i=1;i<=m;++i) for (j=i;j<=m;++j)
	for (g[i][j][n+1]=k=n;k;--k) if (i==j||(a[k][i]==a[k][j]&&g[i][j-1][k]>=k))
	g[i][j][k]=a[k][i]==a[k+1][i]?g[i][j][k+1]:k; else g[i][j][k]=k-1;
	if (f[1][n][1]==m) return puts("0"),0; for (t=1;;++t)
	{
		for (i=1;i<=n;++i) for (j=i;j<=n;++j) for (k=1;k<=m;++k)
		f[i][j][k]=f[i][j][f[i][j][k]+1];
		for (i=1;i<=m;++i) for (j=i;j<=m;++j) for (k=1;k<=n;++k)
		g[i][j][k]=g[i][j][g[i][j][k]+1];
		for (i=1;i<=n;++i) for (j=1;j<=m;++j)
		{
			for (f[i][n+1][j]=j-1,k=i;k<=n;++k)
			for (p=f[i][k][j];p>f[i][k+1][j];--p) g[j][p][i]=max(g[j][p][i],k);
			for (g[j][m+1][i]=i-1,k=j;k<=m;++k)
			for (p=g[j][k][i];p>g[j][k+1][i];--p) f[i][p][j]=max(f[i][p][j],k);
		}
		if (f[1][n][1]==m) return printf("%d",t),0;
	}
	return 0;
}