// includes
#include <bits/stdc++.h>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define vrep(v, i) for(int i = 0; i < (v).size(); i++)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define FI first
#define SE second
#define dump(a, n) for(int i = 0; i < n; i++)cout << a[i] << "\n "[i + 1 != n];
#define dump2(a, n, m) for(int i = 0; i < n; i++)for(int j = 0; j < m; j++)cout << a[i][j] << "\n "[j + 1 != m];
#define bit(n) (1LL<<(n))
#define INT(n) int n; cin >> n;
#define LL(n) ll n; cin >> n;
#define DOUBLE(n) double n; cin >> n;
using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef complex<double> cd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};

// solve
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){os << p.first << " " << p.second; return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}

int h, w;
string s[186];
constexpr int m = 20;
int dp1[201][201][201][2];
int dp2[201][201][201][2];

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(20);
  cin >> h >> w; rep(i, h)cin >> s[i];
  rep(i, h){
    rrep(j, w){
      if(j == w - 1 || s[i][j] != s[i][j+1])dp1[i][i+1][j][0] = j + 1;
      else dp1[i][i+1][j][0] = dp1[i][i+1][j+1][0];
    }
  }
  rep(j, w){
    rrep(i, h){
      if(i == h - 1 || s[i][j] != s[i+1][j])dp2[j][j+1][i][0] = i + 1;
      else dp2[j][j+1][i][0] = dp2[j][j+1][i+1][0];
    }
  }
  rep(h1, h){
    FOR(h2, h1 + 2, h + 1){
      rep(j, w){
        if(s[h2-1][j] == s[h2-2][j])dp1[h1][h2][j][0] = min(dp1[h1][h2-1][j][0], dp1[h2-1][h2][j][0]);
      }
    }
  }
  rep(w1, w){
    FOR(w2, w1 + 2, w + 1){
      rep(i, h){
        if(s[i][w2-1] == s[i][w2-2])dp2[w1][w2][i][0] = min(dp2[w1][w2-1][i][0], dp2[w2-1][w2][i][0]);
      }
    }
  }
  if(dp1[0][h][0][0] == w && dp2[0][w][0][0] == h){
    cout << 0 << endl;
    return 0;
  }
  int res = inf;
  FOR(k, 1, m){
    rep(i, h + 1)rep(j, h + 1)rep(l, w + 1)dp1[i][j][l][k&1] = 0;
    rep(i, w + 1)rep(j, w + 1)rep(l, h + 1)dp2[i][j][l][k&1] = 0;
    rep(h1, h){
      FOR(h2, h1 + 1, h + 1){
        int curr = 1;
        rep(j, w){
          dp1[h1][h2][j][k&1] = dp1[h1][h2][j][(k-1)&1];
          int wb = dp1[h1][h2][j][(k-1)&1];
          if(wb != w)chmax(dp1[h1][h2][j][k&1], dp1[h1][h2][wb][(k-1)&1]);
          curr = max(curr, j+1);
          while(curr <= w){
            int hb = dp2[j][curr][h1][(k-1)&1];
            if(hb == h){
              curr++;
              continue;
            }
            int hb2 = dp2[j][curr][hb][(k-1)&1];
            if(hb2 < h2)break;
            curr++;
          }
          curr--;
          chmax(dp1[h1][h2][j][k&1], curr);
          //cerr << h1 << " " << h2 << " " << j << " " << k << " " << dp1[h1][h2][j][k] << endl;
        }
      }
    }
    rep(w1, w){
      FOR(w2, w1 + 1, w + 1){
        int curr = 1;
        rep(i, h){
          dp2[w1][w2][i][k&1] = dp2[w1][w2][i][(k-1)&1];
          int hb = dp2[w1][w2][i][(k-1)&1];
          if(hb != h)chmax(dp2[w1][w2][i][k&1], dp2[w1][w2][hb][(k-1)&1]);
          curr = max(curr, i+1);
          while(curr <= h){
            int wb = dp1[i][curr][w1][(k-1)&1];
            if(wb == w){
              curr++;
              continue;
            }
            int wb2 = dp1[i][curr][wb][(k-1)&1];
            if(wb2 < w2)break;
            curr++;
          }
          curr--;
          chmax(dp2[w1][w2][i][k&1], curr);
        }
      }
    }
    if(dp1[0][h][0][k&1] == w || dp2[0][w][0][k&1] == h){
      res = k;
      break;
    }
  }

  cout << res << endl;
  return 0;
}


Measured memory_bytes: 120960