#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define trep(i,s,t) for (int i = (t)-1; i >= s; --i)
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("YES");}else{puts("NO");}
const int MX = 186;
const int M = 9;
typedef unsigned char uchar;

char s[MX][MX];
uchar d[2][MX][MX][MX];
uchar e[2][MX][MX][MX];

int main() {
  int h,w;
  scanf("%d%d",&h,&w);
  rep(i,h) scanf("%s",s[i]);
  int o = 0, p = 1;
  rep(si,h)rep(sj,w) {
    char c = s[si][sj];
    int tj = sj;
    trep(ti,si,h) {
      while (tj < w) {
        bool ok = true;
        srep(i,si,ti+1) if (s[i][tj] != c) { ok = false; break;}
        if (!ok) break;
        ++tj;
      }
      d[o][si][sj][ti] = tj;
    }
  }
  rep(si,h)rep(sj,w) {
    char c = s[si][sj];
    int ti = si;
    trep(tj,sj,w) {
      while (ti < h) {
        bool ok = true;
        srep(j,sj,tj+1) if (s[ti][j] != c) { ok = false; break;}
        if (!ok) break;
        ++ti;
      }
      e[o][si][sj][tj] = ti;
    }
  }
  rep(ans,50) {
    // rep(si,h)rep(sj,w)srep(ti,si,h) printf("%d %d %d %d\n",si,sj,ti,d[o][si][sj][ti]);
    // puts("---");
    // rep(si,h)rep(sj,w)srep(tj,sj,w) printf("%d %d %d %d\n",si,sj,tj,e[o][si][sj][tj]);
    // puts("===");
    if (d[o][0][0][h-1] >= w) {
      cout<<ans<<endl;
      return 0;
    }
    swap(o,p);
    rep(si,h)rep(sj,w)rep(ti,h) d[o][si][sj][ti] = d[p][si][sj][ti];
    rep(si,h)rep(sj,w)rep(tj,w) e[o][si][sj][tj] = e[p][si][sj][tj];
    rep(si,h)rep(sj,w) {
      srep(ti,si,h) {
        int tj = int(d[p][si][sj][ti])-1;
        if (tj >= sj) {
          if (tj+1 < w) {
            uchar nj = d[p][si][tj+1][ti];
            maxs(d[o][si][sj][ti],nj);
            maxs(e[o][si][sj][nj-1],uchar(ti+1));
          }
          if (ti+1 < h) {
            uchar ni = e[p][ti+1][sj][tj];
            maxs(d[o][si][sj][ni-1],uchar(tj+1));
            maxs(e[o][si][sj][tj],ni);
          }
        }
      }
      srep(tj,sj,w) {
        int ti = int(e[p][si][sj][tj])-1;
        if (ti >= si) {
          if (tj+1 < w) {
            uchar nj = d[p][si][tj+1][ti];
            maxs(d[o][si][sj][ti],nj);
            maxs(e[o][si][sj][nj-1],uchar(ti+1));
          }
          if (ti+1 < h) {
            uchar ni = e[p][ti+1][sj][tj];
            maxs(d[o][si][sj][ni-1],uchar(tj+1));
            maxs(e[o][si][sj][tj],ni);
          }
        }
      }
      trep(ti,si,h-1) maxs(d[o][si][sj][ti], d[o][si][sj][ti+1]);
      trep(tj,sj,w-1) maxs(e[o][si][sj][tj], e[o][si][sj][tj+1]);
    }
  }
  return 0;
}






















Measured memory_bytes: 25344