#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cerr << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cerr<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 300010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = 1 << 30;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
mt19937 rng; //use it by rng() % mod, shuffle(all(vec), rng)
///////////////////////////////////////////////////////////////////////////////////////////////////

struct Ruiseki2D {
	int N, M;
	vector<vl> B;
	void init(int n, int m) {
		N = n; M = m;
		B = vector<vl>(N + 1, vl(M + 1, 0));
	}
	Ruiseki2D() {}
	Ruiseki2D(int n, int m) { init(n, m); }
	void add(int a, int b, ll v) {
		if(a < 0 || N <= a || b < 0 || M <= b) return;
		B[a + 1][b + 1] += v;
	}
	void sweep() {
		rep(i, 1, N + 1)
			rep(j, 1, M + 1)
				B[i][j] += B[i][j - 1] + B[i - 1][j] - B[i - 1][j - 1];
	}
	ll get(int a, int b, int c, int d) { //[(a, b), (c, d))
		if(a >= c || b >= d) return 0;
		a = max(a, 0); b = max(b, 0); c = min(c, N); d = min(d, M);
		return B[c][d] - B[c][b] - B[a][d] + B[a][b];
	}
};

int N, M;
Ruiseki2D rui;

int dp[2][200][200][200]; // x1, y1, x2: y2 
int ep[2][200][200][200]; // x1, y1, y2: x2

void solve() {
	cin >> N >> M;
	rui.init(N, M);
	rep(i, 0, N) {
		string str; cin >> str;
		rep(j, 0, M) {
			if(str[j] == '#') rui.add(i, j, 1);
		}
	}
	rui.sweep();

	int now = 0, pre = 1;

	rep(q, 0, 20) {
		memset(dp[now], -1, sizeof(dp[now]));
		memset(ep[now], -1, sizeof(ep[now]));
		if(q == 0) {
			rep(i, 0, N + 1) {
				rep(k, i, N + 1) {
					int r = 0;
					rep(j, 0, M + 1) {
						MAX(r, j);
						while(true) {
							if(r < M && (rui.get(i, j, k, r + 1) == 0 || rui.get(i, j, k, r + 1) == (k - i) * (r + 1 - j))) {
								r++;
								// debug("zerodp", i, j, k, r, rui.get(i, j, k, r));
							}
							else break;
						}
						dp[0][i][j][k] = r;
					}
				}
			}
			rep(j, 0, M + 1) {
				rep(l, j, M + 1) {
					int r = 0;
					rep(i, 0, N + 1) {
						MAX(r, i);
						while(true) {
							if(r < N && (rui.get(i, j, r + 1, l) == 0 || rui.get(i, j, r + 1, l) == (r + 1 - i) * (l - j))) {
								r++;
								// debug("zeroep", i, j, r, l, rui.get(i, j, r, l));
							}
							else break;
						}
						ep[0][i][j][l] = r;
					}
				}
			}
		}
		else {
			rep(i, 0, N + 1) {
				rep(k, i, N + 1) {
					int r = 0;
					rep(j, 0, M + 1) {
						int r1 = dp[pre][i][j][k];
						int r2 = dp[pre][i][r1][k];
						MAX(dp[now][i][j][k], r2);
						MAX(r, j);
						while(r < M) {
							int h1 = ep[pre][i][j][r + 1];
							int h2 = ep[pre][h1][j][r + 1];
							if(h2 >= k) r++;
							else break;
						}
						MAX(dp[now][i][j][k], r);
					}
				}
			}
			rep(j, 0, M + 1) {
				rep(l, j, M + 1) {
					int r = 0;
					rep(i, 0, N + 1) {
						int r1 = ep[pre][i][j][l];
						int r2 = ep[pre][r1][j][l];
						MAX(ep[now][i][j][l], r2);
						MAX(r, i);
						while(r < N) {
							int v1 = dp[pre][i][j][r + 1];
							int v2 = dp[pre][i][v1][r + 1];
							if(v2 >= l) r++;
							else break;
						}
						MAX(ep[now][i][j][l], r);

					}
				}
			}
		}
		if(dp[now][0][0][N] == M) {
			cout << q << "\n"; return;
		}
		swap(now, pre);
	}
	cout << inf << "\n";
}


uint32_t rd() {
	uint32_t res;
#ifdef __MINGW32__
	asm volatile("rdrand %0" :"=a"(res) ::"cc");
#else
	res = std::random_device()();
#endif
	return res;
}

int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
    cerr << fixed;
	cerr.precision(6);
	rng.seed(rd());
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
    freopen("in.txt", "rt", stdin);
#endif	
	solve();
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
	return 0;
}



Measured memory_bytes: 125568