/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author majk
 */

#ifndef MAJK_LIB
#define MAJK_LIB

#include <vector>
#include <stack>
#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <iomanip>
#include <set>
#include <functional>
#include <fstream>
#include <algorithm>
#include <cassert>
#include <cmath>
#include <string>
#include <sstream>
#include <queue>
#include <array>
#include <bitset>
using namespace std;

#define x first
#define y second
typedef std::pair<int,int> pii; typedef long long ll; typedef unsigned long long ull; typedef unsigned int ui; typedef pair<ui,ui> puu;

template <typename T, typename U> std::istream&operator>>(std::istream&i, pair<T,U>&p) {i >> p.x >> p.y; return i;}
template<typename T>std::istream&operator>>(std::istream&i,vector<T>&t) {for(auto&v:t){i>>v;}return i;}
template <typename T, typename U> std::ostream&operator<<(std::ostream&o, const pair<T,U>&p) {o << p.x << ' ' << p.y; return o;}
template<typename T>std::ostream&operator<<(std::ostream&o,const vector<T>&t) {if(t.empty())o<<'\n';for(size_t i=0;i<t.size();++i){o<<t[i]<<" \n"[i == t.size()-1];}return o;}
template<typename T> using minheap = priority_queue<T, vector<T>, greater<T>>;
template<typename T> using maxheap = priority_queue<T, vector<T>, less<T>>;
template <typename T> bool in(T a, T b, T c) { return a <= b && b < c; }
ui logceil(ll x) { return x?8*sizeof(ll)-__builtin_clzll(x):0; }

namespace std { template<typename T,typename U>struct hash<pair<T,U>>{hash<T>t;hash<U>u;size_t operator()(const pair<T,U>&p)const{return t(p.x)^(u(p.y)<<7);}}; }
template<typename T,typename F>T bsh(T l,T h,const F&f){T r=-1,m;while(l<=h){m=(l+h)/2;if(f(m)){l=m+1;r=m;}else{h=m-1;}}return r;}
template<typename F> double bshd(double l,double h,const F&f,double p=1e-9){ui r=3+(ui)log2((h-l)/p);while(r--){double m=(l+h)/2;if(f(m)){l=m;}else{h=m;}}return (l+h)/2;}
template<typename T,typename F>T bsl(T l,T h,const F&f){T r=-1,m;while(l<=h){m=(l+h)/2;if(f(m)){h=m-1;r=m;}else{l=m+1;}}return r;}
template<typename F> double bsld(double l,double h,const F&f,double p=1e-9){ui r=3+(ui)log2((h-l)/p);while(r--){double m=(l+h)/2;if(f(m)){h=m;}else{l=m;}}return (l+h)/2;}
template<typename T> T gcd(T a,T b) { if (a<b) swap(a,b); return b?gcd(b,a%b):a; }


template<typename T>class vector2:public vector<vector<T>>{public:vector2(){} vector2(size_t a,size_t b,T t=T()):vector<vector<T>>(a,vector<T>(b,t)){}};
template<typename T>class vector3:public vector<vector2<T>>{public:vector3(){} vector3(size_t a,size_t b,size_t c,T t=T()):vector<vector2<T>>(a,vector2<T>(b,c,t)){}};
template<typename T>class vector4:public vector<vector3<T>>{public:vector4(){} vector4(size_t a,size_t b,size_t c,size_t d,T t=T()):vector<vector3<T>>(a,vector3<T>(b,c,d,t)){}};
template<typename T>class vector5:public vector<vector4<T>>{public:vector5(){} vector5(size_t a,size_t b,size_t c,size_t d,size_t e,T t=T()):vector<vector4<T>>(a,vector4<T>(b,c,d,e,t)){}};


#endif

typedef unsigned char uc;
uc D[186][186][186][18];

class DComplexity {
public:
    void solve(istream& cin, ostream& cout) {
        int H, W; cin >> H >> W;
        vector<string> A(H); cin >> A;
        vector2<int> P(H+1, W+1, 0);
        for (int i = 0; i < H; ++i) {
            for (int j = 0; j < W; ++j) {
                P[i+1][j+1] = P[i+1][j] + P[i][j+1] - P[i][j] + (A[i][j] == '#');
            }
        }

        auto get = [&](int h1, int w1, int h2, int w2) {
            return P[h2+1][w2+1] - P[h2+1][w1] - P[h1][w2+1] + P[h1][w1];
        };

        for (int i = 0; i < H; ++i) {
            for (int j = 0; j < W; ++j) {
                for (int k = j; k < W; ++k) {
                    int s = bsh(i, H-1, [&](int x) { int y = get(i, j, x, k); return y == 0 || y == (x-i+1)*(k-j+1); });
                    if (s == -1) {
                        D[i][j][k][0] = 0;
                    } else {
                        D[i][j][k][0] = s - i + 1;
                    }
                }
            }
        }

        for (int l = 1; l <= 17; ++l) {
            for (int i = 0; i < H; ++i) {
                for (int j = 0; j < W; ++j) {
                    for (int k = j; k < W; ++k) {
                        uc r = D[i][j][k][l-1];
                        if (i+r != H) {
                            D[i][j][k][l] = max(D[i][j][k][l], uc(r + D[i+r][j][k][l-1]));
                        } else {
                            D[i][j][k][l] = max(D[i][j][k][l], r);
                        }

                        if (k!=W-1) {
                            // go right
                            int t = bsh(k+1, W-1, [&](int x) { return D[i][k+1][x][l-1] >= r; });
                            if (t != -1) {
                                D[i][j][t][l] = max(D[i][j][t][l], r);
                            }
                        }
                        if (j != 0) {
                            // go left
                            int t = bsl(0, j-1, [&](int x) { return D[i][x][j-1][l-1] >= r; });
                            if (t != -1) {
                                D[i][t][k][l] = max(D[i][t][k][l], r);
                            }
                        }
                    }
                }
            }

            for (int i = 0; i < H; ++i) {
                for (int len = H; len >= 2; --len) {
                    for (int j = 0; j + len <= H; ++j) {
                        int k = j + len - 1;
                        D[i][j][k-1][l] = max(D[i][j][k-1][l], D[i][j][k][l]);
                        D[i][j+1][k][l] = max(D[i][j+1][k][l], D[i][j][k][l]);
                    }
                }
            }
        }

        for (int l = 0; l <= 16; ++l) {
            if (D[0][0][W-1][l] == H) {
                cout << l << endl;
                return;
            }
        }
    }
};


int main() {
	ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
	DComplexity solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
    return 0;
}


Measured memory_bytes: 113024