#include <bits/stdc++.h>  // clang-format off
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) long long __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
#define inc(...) char __VA_ARGS__; in(__VA_ARGS__)
#define in2(s,t)     rep(i,sz(s)){in(s[i] , t[i]);}
#define in3(s,t,u)   rep(i,sz(s)){in(s[i] , t[i] , u[i]);}
#define in4(s,t,u,v) rep(i,sz(s)){in(s[i] , t[i] , u[i] , v[i]);}
#ifdef ONLINE_JUDGE
  #define rep(i,N) for(int i = 0; i < (int)(N); i++)
  #define repr(i,N) for(int i = (int)(N) - 1; i >= 0; i--)
  #define rep1(i,N) for(int i = 1; i <= (int)(N) ; i++)
  #define repr1(i,N) for(int i = (N) ; (int)(i) > 0 ; i--)
#else
  #define rep(i,N) for(long long i = 0; i < (long long)(N); i++)
  #define repr(i,N) for(long long i = (long long)(N) - 1; i >= 0; i--)
  #define rep1(i,N) for(long long i = 1; i <= (long long)(N) ; i++)
  #define repr1(i,N) for(long long i = (N) ; (long long)(i) > 0 ; i--)
#endif
using namespace std; void solve();
using ll = long long; template<class T = ll> using V = vector<T>;
using vi = V<int>; using vl = V<>; using vvi = V< V<int> >;
using vd = V<double>; using vs = V<string>; using vvl = V< V<> >;
constexpr int inf = 1001001001; constexpr ll infLL = (1LL << 61) - 1;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T,typename U>ll ceil(T a,U b){return (a + b - 1) / b;}
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); for(int i=0;i<s;i++) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}

#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);} while(0)
  #define trcc(v) do { cerr << "name : " << #v << "\n"; int cnt = 0; each(x , v){cerr << (cnt++) << " : "; trc(x); } } while(0)
#else
  #define trc(...)
  #define trca(...)
  #define trcc(...)
  int main(){solve();}
#endif

constexpr ll TEN(int n){ll ret=1,x=10;while(n){if(n&1)ret*=x;x*=x;n>>=1;}return ret;}
#define mem(a, val) memset(a, val, sizeof(a))

struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
using P = pair<ll,ll>; using vp = V<P>;
constexpr int MOD = /**/ 1000000007; //*/ 998244353;
// clang-format on
////////////////////////////////////////////////////

// buildでの構築を忘れない！
template <class T>
struct CumulativeSum2D {
  vector<vector<T> > data;

  CumulativeSum2D(int H, int W) : data(H + 2, vector<int>(W + 2, 0)) {}

  void add(int i, int j, T z) {
    ++i, ++j;
    if (i >= (int)data.size() || j >= (int)data[0].size()) return;
    data[i][j] += z;
  }

  void build() {
    for (int i = 1; i < (int)data.size(); i++) {
      for (int j = 1; j < (int)data[i].size(); j++) {
        data[i][j] += data[i][j - 1] + data[i - 1][j] - data[i - 1][j - 1];
      }
    }
  }

  T query(int i1, int j1, int i2, int j2) {
    return (data[i2][j2] - data[i1][j2] - data[i2][j1] + data[i1][j1]);
  }
};
char c[200][100][200][100] = {};

void solve() {
  auto idx = [](ll a, ll b, ll c, ll d) -> ll {
    return (a << 24) | (b << 16) | (c << 8) | d;
  };
  ini(H, W);
  vs g(H);
  in(g);
  CumulativeSum2D<int> rui(H, W);
  rep(i, H) rep(j, W) rui.add(i, j, g[i][j] == '#');
  rui.build();

  auto dfs = [&](auto rec, int x1, int y1, int x2, int y2) -> int {
    if (rui.query(x1, y1, x2, y2) == 0) return 0;
    if (rui.query(x1, y1, x2, y2) == (x2 - x1) * (y2 - y1)) return 0;
    ll id = idx(x1, y1, x2, y2);
    if(x2-x1<100 && y2-y1<100 &&  c[x1][x2-x1][y1][y2-y1] !=0)
      return c[x1][x2-x1][y1][y2-y1];

    int ret = inf;

    int l = x1, r = x2;
    while (l + 1 < r) {
      int m = (l + r) / 2;
      int v1 = rec(rec, x1, y1, m, y2);
      int v2 = rec(rec, m, y1, x2, y2);
      if (ret < max(v1, v2)) break;
      amin(ret, max(v1, v2));
      if (v1 == v2) break;
      if (v1 < v2)
        l = m;
      else
        r = m;
    }

    int u = y1, d = y2;
    while (u + 1 < d) {
      int m = (u + d) / 2;
      int v1 = rec(rec, x1, y1, x2, m);
      int v2 = rec(rec, x1, m, x2, y2);
      if (ret < max(v1, v2)) break;
      amin(ret, max(v1, v2));
      if (v1 == v2) break;
      if (v1 < v2)
        u = m;
      else
        d = m;
    }
    if(r - l < 100 && d - u < 100)
      c[x1][x2-x1][y1][y2-y1] = ret + 1;
    return ret + 1;
  };

  out(dfs(dfs, 0, 0, H, W));
}

Measured memory_bytes: 360832