#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <iostream>
#include <string>
#include <vector>
#include <list>
#include <utility>
#include <algorithm>
#include <functional>
#include <cmath>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <limits>
#include <numeric>
#include <valarray>
#include <fstream>

using namespace std;
typedef unsigned int uint;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<LL, LL> PP;
#define REP(i, a, n) for(LL i = (a), i##_max = (n); i < i##_max; ++i)
#define REM(i, a, n) for(LL i = (LL)(n) - 1, i##min = (a); i >= i##min; --i)
#define ALL(arr) (arr).begin(), (arr).end()
#define FLOAT fixed << setprecision(16)
#define SPEEDUP {cin.tie(NULL); ios::sync_with_stdio(false);}
const int INF = 0x3FFFFFFF;
const LL INFLL = 0x3FFFFFFF3FFFFFFF;
const double INFD = 1.0e+308;
const double EPS = 1.0e-9;

void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T, class U>
istream& operator>>(istream& ist, pair<T, U>& right) { return ist >> right.first >> right.second; }
template <class T, class U>
ostream& operator<<(ostream& ost, const pair<T, U>& right) { return ost << right.first << ' ' << right.second; }
template <class T, class TCompatible, size_t N>
void Fill(T(&dest)[N], const TCompatible& val) { fill(dest, dest + N, val); }
template <class T, class TCompatible, size_t M, size_t N>
void Fill(T(&dest)[M][N], const TCompatible& val) { for (int i = 0; i < M; ++i) Fill(dest[i], val); }
template<class T>
T Compare(T left, T right) { return left > right ? 1 : (left < right ? -1 : 0); }
istream& Ignore(istream& ist) { string s; ist >> s; return ist; }
bool Inside(int i, int j, int h, int w) { return i >= 0 && i < h && j >= 0 && j < w; }
template <class T>
T Next() { T buf; cin >> buf; return buf; }

#ifdef ONLY_MY_ENVIR
#include "IntMod.h"
#include "BinaryMatrix.h"
#include "BIT.h"
#include "Factorization.h"
#include "FlowSolver.h"
#include "Graph.h"
#include "LazySegmentTree.h"
#include "Math.h"
#include "Matrix.h"
#include "MinMax.h"
#include "Position.h"
#include "Range.h"
#include "Rational.h"
#include "SegmentTree.h"
#include "SegmentTree2D.h"
#include "SuffixArray.h"
#include "Tree.h"
#include "UnionFind.h"
#endif

#ifdef __GNUC__
typedef __int128 LLL;
istream& operator>> (istream& ist, __int128& val) { LL tmp;  ist >> tmp; val = tmp; return ist; }
ostream& operator<< (ostream& ost, __int128 val) { LL tmp = val; ost << tmp; return ost; }
#endif

#if 1234567891
#include <array>
#include <random>
#include <unordered_set>
#include <unordered_map>
template<typename T>
using PriorityQ = priority_queue<T, vector<T>, greater<T> >;
// template <class T>
// auto Is(const T& value) { return [value](const auto& comparand) -> bool { return comparand == value; }; }
#endif

const int K = 16;
int H, W;
string S[185];
int DP[2][186][186][186];
int DQ[2][186][186][186];
int P[186][186];

void pre() {
	REP(i, 0, H) {
		REP(j, 0, W) {
			P[i + 1][j + 1] = S[i][j] == '#';
		}
	}

	REP(i, 1, H + 1) {
		REP(j, 0, W) {
			P[i][j + 1] += P[i][j];
		}
	}

	REP(j, 1, W + 1) {
		REP(i, 0, H) {
			P[i + 1][j] += P[i][j];
		}
	}
}

int main() {
	cin >> H >> W;
	REP(i, 0, H) {
		cin >> S[i];
	}

	pre();
	REP(li, 0, H) {
		REP(ri, li + 1, H + 1) {
			int rj = 0;
			REP(lj, 0, W + 1) {
				rj = max<int>(rj, lj);
				while (rj <= W) {
					int val = P[ri][rj] - P[li][rj] - P[ri][lj] + P[li][lj];
					if (val != 0 && val != (ri - li) * (rj - lj)) break;
					++rj;
				}
				DP[0][li][ri][lj] = rj - 1;
			}
		}
	}

	REP(lj, 0, W) {
		REP(rj, lj + 1, W + 1) {
			int ri = 0;
			REP(li, 0, H + 1) {
				ri = max<int>(ri, li);
				while (ri <= H) {
					int val = P[ri][rj] - P[li][rj] - P[ri][lj] + P[li][lj];
					if (val != 0 && val != (ri - li) * (rj - lj)) break;
					++ri;
				}
				DQ[0][lj][rj][li] = ri - 1;
			}
		}
	}

	int ans = 0;
	if (DP[0][0][H][0] == W) {
		ans = 0;
		goto end;
	}

	REP(k, 0, K) {
		int x = k % 2;
		int y = (k + 1) % 2;

		Fill(DP[y], 0);
		Fill(DQ[y], 0);
		REP(li, 0, H) {
			REP(ri, li + 1, H + 1) {
				REP(lj, 0, W + 1) {
					int mj = DP[x][li][ri][lj];
					DP[y][li][ri][lj] = max(DP[y][li][ri][lj], DP[x][li][ri][mj]);
					int rj = DP[y][li][ri][lj];
					DQ[y][lj][rj][li] = max(DQ[y][lj][rj][li], (int)ri);
				}
			}
		}

		REM(dj, 1, W + 1) {
			REP(lj, 0, W) {
				int rj = lj + dj;
				if (rj > W) continue;
				REP(li, 0, H + 1) {
					if (lj >= 1) DQ[y][lj][rj][li] = max(DQ[y][lj][rj][li], DQ[y][lj - 1][rj][li]);
					if (rj < W) DQ[y][lj][rj][li] = max(DQ[y][lj][rj][li], DQ[y][lj][rj + 1][li]);
				}
			}
		}

		REP(lj, 0, W) {
			REP(rj, lj + 1, W + 1) {
				REP(li, 0, H + 1) {
					int mi = DQ[x][lj][rj][li];
					DQ[y][lj][rj][li] = max(DQ[y][lj][rj][li], DQ[x][lj][rj][mi]);
					int ri = DQ[y][lj][rj][li];
					DP[y][li][ri][lj] = max(DP[y][li][ri][lj], (int)rj);
				}
			}
		}
		
		REM(di, 1, H + 1) {
			REP(li, 0, H) {
				int ri = li + di;
				if (ri > H) continue;
				REP(lj, 0, W + 1) {
					if (li >= 1) DP[y][li][ri][lj] = max(DP[y][li][ri][lj], DP[y][li - 1][ri][lj]);
					if (ri < H) DP[y][li][ri][lj] = max(DP[y][li][ri][lj], DP[y][li][ri + 1][lj]);
				}
			}
		}

		if (DP[y][0][H][0] == W) {
			ans = k + 1;
			break;
		}
	}
end:
	cout << ans << endl;
	return 0;
}


Measured memory_bytes: 100992