//clever
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;

random_device(rd);
mt19937 rng(rd());
const long long FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();

struct custom_hash
{
	template<class T>
	unsigned long long operator()(T v) const
	{
		unsigned long long x = v;
		x += FIXED_RANDOM;
		// x += 11400714819323198485ull;
		// x = (x ^ (x >> 30)) * 13787848793156543929ull;
		x = (x ^ (x >> 27)) * 10723151780598845931ull;
		return x ^ (x >> 31);
	}
};

template<class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<class T, class U> using hash_table = gp_hash_table<T, U, custom_hash>;

template<class T>
T randomize(T mod)
{
	return (uniform_int_distribution<T>(0, mod - 1))(rng);
}
template<class T>
void readi(T &x)
{
	x = 0;
	bool negative = false;
	char c = ' ';
	while (c < '-')
	{
		c = getchar();
	}
	if (c == '-')
	{
		negative = true;
		c = getchar();
	}
	while (c >= '0')
	{
		x = x * 10 + (c - '0');
		c = getchar();
	}
	if (negative)
	{
		x = -x;
	}
}
template<class T>
void printi(T output)
{
	if (output == 0)
	{
		putchar('0');
		return;
	}
	if (output < 0)
	{
		putchar('-');
		output = -output;
	}
	int buf[20], n = 0;
	while(output)
	{
		buf[n] = ((output % 10));
		output /= 10;
		n++;
	}
	for (n--; n >= 0; n--)
	{
		putchar(buf[n] + '0');
	}
	return;
}
template<class T>
void ckmin(T &a, T b)
{
	a = min(a, b);
}
template<class T>
void ckmax(T &a, T b)
{
	a = max(a, b);
}
long long expo(long long a, long long e, long long mod)
{
	return ((e == 0) ? 1 : ((expo(a * a % mod, e >> 1, mod)) * ((e & 1) ? a : 1) % mod));
}
template<class T, class U>
void nmod(T &x, U mod)
{
	if (x >= mod) x -= mod;
}
template<class T>
T gcd(T a, T b)
{
	return (b ? gcd(b, a % b) : a);
}

#define y0 ___y0
#define y1 ___y1
#define MP make_pair
#define PB push_back
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define DBG(x) cerr << #x << " = " << (x) << endl
#define SZ(x) ((int) ((x).size()))
#define FOR(i, a, b) for (auto i = (a); i < (b); i++)
#define FORD(i, a, b) for (auto i = (a) - 1; i >= (b); i--)
#define ALL(x) (x).begin(), (x).end()

const long double PI = 4.0 * atan(1.0);
const long double EPS = 1e-9;

#define MAGIC 347
#define SINF 10007
#define CO 1000007
#define INF 1000000007
#define BIG 1000000931
#define LARGE 1696969696967ll
#define GIANT 2564008813937411ll
#define LLINF 2696969696969696969ll
#define MAXN 188

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pdd;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<ld> vd;
typedef vector<pii> vpi;
typedef vector<pll> vpl;
typedef vector<pdd> vpd;

int N, M;
bitset<MAXN> grid[MAXN];
short hor[MAXN][MAXN][MAXN][17], vert[MAXN][MAXN][MAXN][17];
int ans;

int32_t main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	// cout << fixed << setprecision(10);
	// cerr << fixed << setprecision(10);
	// if (fopen("file.in", "r"))
	// {
	// 	freopen ("file.in", "r", stdin);
	// 	freopen ("file.out", "w", stdout);
	// }
	cin >> N >> M;
	FOR(i, 0, N)
	{
		string temps; cin >> temps;
		FOR(j, 0, M)
		{
			grid[i][j] = (temps[j] == '.');
		}
	}
	FORD(k, M, 0)
	{
		FOR(i, 0, N)
		{
			bool ok = true, ok1 = (k != M - 1);
			FOR(j, i, N)
			{
				//for this edge (i....j) at this M, how far right can we go to have complexity 0
				if (grid[i][k] != grid[j][k]) ok = false;
				if (grid[j][k] != grid[j][k + 1]) ok1 = false;
				hor[i][j][k][0] = (ok ? (1 + (ok1 ? hor[i][j][k + 1][0] : 0)) : 0);
			}
		}
	}
	FORD(k, N, 0)
	{
		FOR(i, 0, M)
		{
			bool ok = true, ok1 = (k != N - 1);
			FOR(j, i, M)
			{
				if (grid[k][i] != grid[k][j]) ok = false;
				if (grid[k][j] != grid[k + 1][j]) ok1 = false;
				vert[i][j][k][0] = (ok ? (1 + (ok1 ? vert[i][j][k + 1][0] : 0)) : 0);
			}
		}
	}
	FOR(m, 1, 17)
	{
		FORD(k, M, 0)
		{
			FOR(i, 0, N)
			{
				FOR(j, i, N)
				{
					//for this edge (i....j) at this M, how far right can we go to have complexity 0
					short c = hor[i][j][k][m - 1];
					ckmax(hor[i][j][k][m], (short) (c + ((k + c == M) ? 0 : hor[i][j][k + c][m - 1])));
					if (hor[i][j][k][m] > 0)
					ckmax(vert[k][k + hor[i][j][k][m] - 1][i][m], (short) (j - i + 1));
					// cerr << k << ' ' << i << ' ' << j << endl;
					//vertical: i...j horizontal: k...k + hor[i][j][k][m] - 1
				}
			}
		}
		FORD(k, N, 0)
		{
			FOR(i, 0, M)
			{
				FOR(j, i, M)
				{
					short c = vert[i][j][k][m - 1];
					ckmax(vert[i][j][k][m], (short) (c + ((k + c == N) ? 0 : vert[i][j][k + c][m - 1])));
					if (vert[i][j][k][m] > 0)
					ckmax(hor[k][k + vert[i][j][k][m] - 1][i][m], (short) (j - i + 1));
					//horizontal: i...j vertical: k...k + vert[i][j][k][m] - 1
				}
			}
		}
		FORD(k, M, 0)
		{
			FOR(i, 0, N)
			{
				FORD(j, N - 1, 0)
				{
					//for this edge (i....j) at this M, how far right can we go to have complexity 0
					ckmax(hor[i][j][k][m], hor[i][j + 1][k][m]);
					// cerr << k << ' ' << i << ' ' << j << endl;
					//vertical: i...j horizontal: k...k + hor[i][j][k][m] - 1
				}
			}
		}
		FORD(k, N, 0)
		{
			FOR(i, 0, M)
			{
				FORD(j, M - 1, 0)
				{
					ckmax(vert[i][j][k][m], vert[i][j + 1][k][m]);
					//horizontal: i...j vertical: k...k + vert[i][j][k][m] - 1
				}
			}
		}
	}
	FOR(m, 0, 17)
	{
		if (hor[0][N - 1][0][m] == M || hor[0][M - 1][0][m] == N)
		{
			cout << m << '\n';
			return 0;
		}
	}
	assert(false);
	cout << "-1\n";
	// cerr << "time elapsed = " << (clock() / (CLOCKS_PER_SEC / 1000)) << " ms" << endl;
	return 0;
}
/* READ READ READ
* int overflow, maxn too small, special cases (n=1?, two distinct?), cin.tie() interactive
* reread the problem, try small cases
* note down possible sources of error as you go
* do smth instead of nothing
*/


Measured memory_bytes: 438528