/*
                                                                                                     
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                                          .+:--.......--:+`                                         
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                                         .o:--..`` ``..--:o`                                        
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                                       `/y+o/---....---:+o.                                         
                                   `...````-os+/:---:/+o/--.`                                       
              `-/+++++/:.      `...`       :h+d+oooo+/+-`   ...                                     
            `/++//:::://++-`....`         -.`//````````:`     `..`                                  
           `o+/::------://o/`           `-` -.          -`       `..`                               
 `---.-o/:./o/::-..``..-ЗАПУСКАЕМ      ..  ..            -`        `...       ``..``                
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          `:s+/:--....-::/+s-`      .-   `-                -`           -///:--------:/:`           
           ./s+//:::::://oo-``..НЕЙРОННУЮ: СЕТЬ:::::::-`РАБОТЯГИ        `+:--........--:/`          
            .:ooo+++++osso-`    `.:-...`/` ./::-------:/:`   -`         :+--..``````.--:+:...-+:-`  
             `.-/+++++/+-.-`    -.   ``:so:/:--.......--:+`  `-```````o+/+--..`````..--:o/-..:s+:.  
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                `.---..``-..-    .-/..`````-oo+/:::::/+o+-        `-``-`  `-.  ````                 
             `:++++/+++++-  ..``.-/:`      /y-:/++o++/:.`..`       ./.   `-                         
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       ``  -o//:--....-::/++` -.-`   `-`.-`                 `..`..`  `-.-                           
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  `````/:..+o/::-..``.--:/+s. ..-`   `-``-`                 ..` `-`  `-`-`                          
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            `./+oooooooo+/.`-    .-:...`.. .//:-------://`        `- `..` `:.                       
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                          -.-`.-    .-   `+:--..`````..--//`    `-       /s-//::::::::.             
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                        -` .-`.-`     -.`-o/--...```..--:+/.``-:....``:-.+:--....`...--:+`          
                       ..`-.   `-.   ``:os:o/:---...---:++.  `-     ``///+:-..``````.--:+-````-.`   
              `.:///////.-`      .:-..` -``-+o+/:::::/+o/.  `-         `:+:-..`````..--:o/:--/ys+-  
            `-++///////+o/. ``....`-.    :` `.:++++++/:.`  .-           -o/---......---/o.   `.`    
           `++//:-----::/+o:..`     .-`   :    ```````    .-           `+so+:--------:++-`          
  `````:-``:o/::-..`..--:/+o`         -.  `-             .-          `../../+o+////+o+:.`           
  -----syo/o+/:--.```..-://s.          .-` `-           .-        `...     ``-:////:-``             
       .` `/s//:--....-::/+s.            -. `-`        .-       `..`                                
           .+o+/:::--:://+s/-..`          .::+y  ```  .-     `..`                                   
            ./oo++////+oso-`   `....       :y-+:::::::/`   ...                                      
             `.:+oooooo/-`         `....-. .//:-------:/:-.`                                        
                ``...``                 /+:+:--.......--:+`                                         
                                         `+:--..`````..--//`                                        
                                         .o:--..`` ``..--:o`                                        
                                         .+/--...```..--:+/`                                        
                                         `-o/:---...---:++.                                         
                                          `-+o+/:---:/+o/.                                          
                                            `.:+oooo+/-.`                                           
                                               ``````                                               
*/

#ifdef aimbot
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("unroll-loops")
#endif

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <queue>
#include <ostream>
#include <istream>
#include <typeinfo>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <limits>
#include <fstream>
#include <array>
#include <list>
#include <bitset>
#include <functional>
#include <random>
#include <cstring>
#include <chrono>

#define random escape__from__random__aetuhoetnuhshe
#define mt make_tuple
#define x first
#define y second
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define umap unordered_map
#define uset unordered_set
#define elif else if
#define len(v) ((int)v.size())
#define f(i, n) for (int i = 0; i < (n); i++)
#define rof(i, n) for (int i = ((n) - 1); i >= 0; i--)
#define apply(v, act) for (auto &x : v) { act; }
#define log(args...) {string s = #args;deque<string> deq;\
string buf = "";int bal = 0;for (char c : s) {\
if (c == '(' || c == '[' || c == '{') {bal++;\
} else if (c == ')' || c == ']' || c == '}') {\
bal--;} else {if (bal == 0) {if (c == ',') {\
deq.pb(buf);buf = "";} else {if (c != ' ') {\
buf += c;}}}}}if (!buf.empty()) {deq.pb(buf);}\
smart_io::precall_print();smart_io::_print(deq, args);}

inline int min(const int &x, const int &y) { return (((y-x)>>(32-1))&(x^y))^x; }
inline int max(const int &x, const int &y) { return (((y-x)>>(32-1))&(x^y))^y; }
inline long long min(const long long &x, const long long &y) { return (((y-x)>>(64-1))&(x^y))^x; }
inline long long max(const long long &x, const long long &y) { return (((y-x)>>(64-1))&(x^y))^y; }

#define print    \
smart_io::precall_print(); \
cout,

#define scan cin,

#ifdef fast_allocator
const int MAXMEM = 200 * 1000 * 1024;
char _memory[MAXMEM];
size_t _ptr = 0;
void* operator new(size_t _x) { _ptr += _x; assert(_ptr < MAXMEM); return _memory + _ptr - _x; }
void operator delete (void*) noexcept {}
#endif

using namespace std;

char string_in_buffer[(int)260];


void fast_scan(int &x) { scanf("%d", &x); }
void fast_scan(long long &x) { scanf("%lld", &x); }
void fast_scan(unsigned long long &x) { scanf("%llu", &x); }
void fast_scan(double &x) { scanf("%lf", &x); }
void fast_scan(long double &x) { scanf("%Lf", &x); }
void fast_scan(char &x) {
	scanf("%c", &x);
	if (x == '\n') {
		fast_scan(x);
	}
}
void fast_scan(string &x) {
	scanf("%s", string_in_buffer);
	x = string(string_in_buffer);
}

template<class TFirst, class TSecond>
void fast_scan(pair<TFirst, TSecond> &p) {
	fast_scan(p.first);
	fast_scan(p.second);
}

template <class T>
void fast_scan(vector<T> &v) {
	for (auto &x : v) fast_scan(x);
}

void fast_print(const int &x) { printf("%d", x); }
void fast_print(const unsigned int &x) { printf("%u", x); }
void fast_print(const long long &x) { printf("%lld", x); }
void fast_print(const unsigned long long &x) { printf("%llu", x); }
void fast_print(const double &x) { printf("%.15lf", x); }
void fast_print(const long double &x) { printf("%.15Lf", x); }
void fast_print(const char &x) { printf("%c", x); };
void fast_print(const string &x) { printf("%s", x.c_str());}
void fast_print(const char v[]) { fast_print((string)v); }

template<class TFirst, class TSecond>
void fast_print(const pair<TFirst, TSecond> &p) {
	fast_print(p.first);
	fast_print(' ');
	fast_print(p.second);
}

template <class T>
void fast_print(const vector<T> &v) {
	if (v.empty()) return;
	fast_print(v[0]);
	for (int i = 1; i < v.size(); i++) {
		fast_print(' ');
		fast_print(v[i]);
	}
}

template <class T>
void fast_print(const vector<vector<T>> &v) {
	if (v.empty()) return;
	fast_print(v[0]);
	for (int i = 1; i < v.size(); i++) {
		fast_print('\n');
		fast_print(v[i]);
	}
}

template <class T>
void fast_print(const T &v) {
	for (const auto &x : v) {
		fast_print(x);
		fast_print(' ');
	}
}


using namespace std;


namespace smart_io {
	string print_start = "";
	string sep = " ";
	bool first_print = false;
	
	void precall_print() {
		fast_print(print_start);
		print_start = "\n";
		first_print = true;
	}
	
	void _print(deque<string>) {}
	template<class T, class... Args>
	void _print(deque<string> names, T elem, Args... args) {
		if (!first_print) {
			fast_print("\n");
		} else {
			first_print = false;
		}
		fast_print(names.front());
		fast_print(" = ");
		fast_print(elem);
		names.pop_front();
		_print(names, args...);
	}
} //namespace smart_io


template <class T>
ostream &operator,(ostream &os, const T &object) {
	if (!smart_io::first_print) {
		fast_print(smart_io::sep);
	} else {
		smart_io::first_print = false;
	}
	fast_print(object);
	return os;
}

template <class T>
istream &operator,(istream &is, T &object) {
	fast_scan(object);
	return is;
}

namespace random {
	using namespace std::chrono;
	mt19937 rng(duration_cast< milliseconds >(
											  system_clock::now().time_since_epoch()
											  ).count());
	uniform_real_distribution<> prob_dist(0.0, 1.0);
};

namespace typedefs {
	typedef long long ll;
	typedef unsigned long long ull;
	typedef pair<int, int> pii;
	typedef long double ld;
}

namespace numbers_operation {
	template<class T>
	T floor_mod(T a, T b) {
		if (a >= 0 && b >= 0) return a % b;
		if (a <= 0 && b <= 0) return a % b;
		return abs(b) - (abs(a) % abs(b));
	}
}

using namespace numbers_operation;
using namespace typedefs;
using namespace random;

const int N = 186;

string v[N];
int pr[N][N];

int __sm(int x, int y, int w, int h) {
	int sm = pr[x + w - 1][y + h - 1] - pr[x - 1][y + h - 1]
		- pr[x + w - 1][y - 1] + pr[x - 1][y - 1];
	return sm;
}

bool zero(int x, int y, int w, int h) {
	if (w == 0 || h == 0) return true;
	x++, y++;
	int sm = pr[x + w - 1][y + h - 1] - pr[x - 1][y + h - 1]
		- pr[x + w - 1][y - 1] + pr[x - 1][y - 1];
	return sm == 0 || sm == w * h;
}

int dpL[2][187][187][187];
int dpR[2][187][187][187];

signed main(signed argc, char *argv[]) {
	int n, m;
	scan n, m;

	f(i, n) {
		string s;
		scan s;
		f(j, m) {
			pr[i + 1][j + 1] = (s[j] == '.');
			pr[i + 1][j + 1] += pr[i][j + 1];
			pr[i + 1][j + 1] += pr[i + 1][j];
			pr[i + 1][j + 1] -= pr[i][j];
		}
	}
	for (int i = 0; i < n; i++)
		for (int j = 0; j < m; j++) {
			int cur = m;
			for (int k = i; k < n; k++) {
				while (!zero(i, j, k - i + 1, cur - j))
					cur--;
				dpL[0][i][j][k] = cur;
			}	
		}
	for (int i = 0; i < n; i++)
		for (int j = 0; j < m; j++) {
			int cur = n;
			for (int k = j; k < m; k++) {
				while (!zero(i, j, cur - i, k - j + 1))
					cur--;
				dpR[0][i][j][k] = cur;
			}
		}
	for (int c = 1; c < 26; c++) {
		if (dpL[0][0][0][n - 1] == m || dpR[0][0][0][m - 1] == n) {
			print c - 1;
			return 0;
		}
		for (int i = 0; i < n; i++) 
			for (int j = 0; j < m; j++) {
				int ver = m - 1;
				for (int k = i; k < n; k++) {
					int sub = dpL[0][i][j][k];
					if (sub < m) sub = dpL[0][i][sub][k];
					dpL[1][i][j][k] = sub;

					if (k > i) {
						while (ver >= j) {
							int up = dpR[0][i][j][ver];
							if (up <= k) up = dpR[0][up][j][ver];
							if (up > k) break;
							ver--;
						}
						dpL[1][i][j][k] = max(dpL[1][i][j][k], ver + 1);
					}
				}
			}
		for (int i = 0; i < n; i++)
			for (int j = 0; j < m; j++) {
				int hor = n - 1;
				for (int k = j; k < m; k++) {
					int sub = dpR[0][i][j][k];
					if (sub < n) sub = dpR[0][sub][j][k];
					dpR[1][i][j][k] = sub;

					if (k > j) {
						while (hor >= i) {
							int right = dpL[0][i][j][hor];
							if (right <= k) right = dpL[0][i][right][hor];
							if (right > k) break;
							hor--;
						}
						dpR[1][i][j][k] = max(dpR[1][i][j][k], hor + 1);
					}
				}
			}
		swap(dpL[0], dpL[1]);
		swap(dpR[0], dpR[1]);
	}
}


Measured memory_bytes: 102528