#include <iostream>
#include <string>
#include <cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<iomanip>
#define _USE_MATH_DEFINES
#include <math.h>
#include <functional>
#include<complex>
using namespace std;

#define rep(i,x) for(ll i=0;i<x;i++)
#define repn(i,x) for(ll i=1;i<=x;i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
	if (a > b) { return a; }
	return b;
}

ll min(ll a, ll b) {
	if (a > b) { return b; }
	return a;
}

ll gcd(ll a, ll b) {
	if (b == 0) { return a; }
	if (a < b) { return gcd(b, a); }
	return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

struct edge {
	ll ind;
	ll fr;
	ll to;
	ll d;
};

class mint {
	long long x;
public:
	mint(long long x = 0) : x((x% MOD + MOD) % MOD) {}
	mint operator-() const {
		return mint(-x);
	}
	mint& operator+=(const mint& a) {
		if ((x += a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator-=(const mint& a) {
		if ((x += MOD - a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator*=(const  mint& a) {
		(x *= a.x) %= MOD;
		return *this;
	}
	mint operator+(const mint& a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint& a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint& a) const {
		mint res(*this);
		return res *= a;
	}

	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime MOD
	mint inv() const {
		return pow(MOD - 2);
	}
	mint& operator/=(const mint& a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint& a) const {
		mint res(*this);
		return res /= a;
	}

	friend ostream& operator<<(ostream& os, const mint& m) {
		os << m.x;
		return os;
	}
};

mint pw(mint a, ll b) {
	if (b == 0) { return 1; }
	mint ret = pw(a, b >> 1);
	ret *= ret;
	if (b & 1) { ret *= a; }
	return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

//////////////////////////////////////
int main() {
	ll H, W;
	cin >> H >> W;

	vector<vector<char>> A(H + 1, vector<char>(W + 1));
	repn(i, H)repn(j, W) cin >> A[i][j];

	vector<vector<vector<unsigned char>>> dpx(H + 1, vector<vector<unsigned char>>(H + 1, vector<unsigned char>(W + 2, 0)));
	vector<vector<vector<unsigned char>>> dpy(W + 1, vector<vector<unsigned char>>(W + 1, vector<unsigned char>(H + 2, 0)));

	repn(i, H)for (ll j = i; j <= H; j++) {
		vector<bool> b(W + 2, 1);

		repn(k, W) {
			for (ll l = i; l <= j; l++) {
				if (A[l][k] != A[i][k]) { b[k] = 0; }
			}
		}
		b[W + 1] = 0;

		ll x = W + 1;
		for (ll k = W + 1; k >= 1; k--) {
			if (b[k] == 0) { x = k; }
			else if (k < W && A[i][k] != A[i][k + 1]) { x = k + 1; }
			dpx[i][j][k] = x;

			//cout << i << " " << j << " " << k << " " << dpx[i][j][k][0] << endl;
		}
	}

	repn(i, W)for (ll j = i; j <= W; j++) {
		vector<bool> b(H + 2, 1);

		repn(k, H) {
			for (ll l = i; l <= j; l++) {
				if (A[k][l] != A[k][i]) { b[k] = 0; }
			}
		}
		b[H + 1] = 0;

		ll x = H + 1;
		for (ll k = H + 1; k >= 1; k--) {
			if (b[k] == 0) { x = k; }
			else if (k < H && A[k][i] != A[k + 1][i]) { x = k + 1; }
			dpy[i][j][k] = x;

			//cout << i << " " << j << " " << k << " " << dpy[i][j][k][0] << endl;
		}
	}

	rep(t, 17) {
		if (dpx[1][H][1] > W) { cout << t << endl; return 0; }

		repn(i, H)for (ll j = i; j <= H; j++) {

			repn(k, W + 1) {
				ll y = dpx[i][j][dpx[i][j][k]];

				y = max(y, dpx[i][j][k - 1]);
				//cout << y << endl;

				dpx[i][j][k] = W + 1;

				while (y <= W) {
					if (dpy[k][y][dpy[k][y][i]] > j) {
						y++;
					}

					else { dpx[i][j][k] = y; break; }
				}

				//cout << i << " " << j << " " << k << " " << t << " " << dpx[i][j][k][t] << endl;
			}

		}

		repn(i, W)for (ll j = i; j <= W; j++) {

			repn(k, H + 1) {
				ll y = dpy[i][j][dpy[i][j][k]];
				y = max(y, dpy[i][j][k - 1]);

				dpy[i][j][k] = H + 1;
				while (y <= H) {
					if (dpx[k][y][i] > j) {
						y++;
					}

					else { dpy[i][j][k] = y; break; }
				}

				//cout << i << " " << j << " " << k << " " << t << " " << dpy[i][j][k][t] << endl;
			}

		}

	}
}


Measured memory_bytes: 19192