//Author:xht37
#include <bits/stdc++.h>

#define ui unsigned int
#define ll long long
#define ul unsigned ll
#define ld long double

#define pi pair <int, int>
#define fi first
#define se second
#define mp make_pair

#define ls (p << 1)
#define rs (ls | 1)
#define md ((t[p].l + t[p].r) >> 1)

#define vi vector <int>
#define pb push_back
#define pq priority_queue

#define dbg(x) cerr << #x" = " << x << endl
#define debug(...) fprintf(stderr, __VA_ARGS__)

#define fl(x) freopen(x".in", "r", stdin), freopen(x".out", "w", stdout)

using namespace std;

namespace io {
	const int SI = 1 << 21 | 1;
	char IB[SI], *IS, *IT, OB[SI], *OS = OB, *OT = OS + SI - 1, c, ch[100];
	int f, t;
	#define gc() (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin), IS == IT ? EOF : *IS++) : *IS++)
	inline void flush() {
		fwrite(OB, 1, OS - OB, stdout), OS = OB;
	}
	inline void pc(char x) {
		*OS++ = x;
		if (OS == OT) flush();
	}

	template <class I>
	inline void rd(I &x) {
		for (f = 1, c = gc(); c < '0' || c > '9'; c = gc()) if (c == '-') f = -1;
		for (x = 0; c >= '0' && c <= '9'; x = (x << 3) + (x << 1) + (c & 15), c = gc());
		x *= f;
	}
	template <class I>
	inline void rd(I &x, I &y) {
		rd(x), rd(y);
	}
	template <class I>
	inline void rd(I &x, I &y, I &p) {
		rd(x), rd(y), rd(p);
	}
	template <class I>
	inline void rda(I *a, int n) {
		for (int i = 1; i <= n; i++) rd(a[i]);
	}
	inline void rdc(char &c) {
		for (c = gc(); c < 33 || c > 126; c = gc());
	}
	inline void rds(char *s, int &n) {
		for (c = gc(); c < 33 || c > 126; c = gc());
		for (n = 0; c >= 33 && c <= 126; s[++n] = c, c = gc());
		s[n+1] = '\0';
	}
	inline void rds(string &s) {
		for (c = gc(); c < 33 || c > 126; c = gc());
		for (s.clear(); c >= 33 && c <= 126; s.pb(c), c = gc());
	}

	template <class I>
	inline void print(I x, char k = '\n') {
		if (!x) pc('0');
		if (x < 0) pc('-'), x = -x;
		while (x) ch[++t] = x % 10 + '0', x /= 10;
		while (t) pc(ch[t--]);
		pc(k);
	}
	template <class I>
	inline void print(I x, I y) {
		print(x, ' '), print(y);
	}
	template <class I>
	inline void print(I x, I y, I p) {
		print(x, ' '), print(y, ' '), print(p);
	}
	template <class I>
	inline void printa(I *a, int n) {
		for (int i = 1; i <= n; i++) print(a[i], " \n"[i==n]);
	}
	inline void printc(char c) {
		pc(c);
	}
	inline void prints(char *s, int n) {
		for (int i = 1; i <= n; i++) pc(s[i]);
		pc('\n');
	}
	inline void prints(string s) {
		int n = s.length();
		while (t < n) pc(s[t++]);
		pc('\n'), t = 0;
	}
	struct Flush {
		~Flush() {
			flush();
		}
	} flusher;
}
using io::rd;
using io::rda;
using io::rdc;
using io::rds;
using io::print;
using io::printa;
using io::printc;
using io::prints;

const int N = 188;
int n, m, w;
char a[N][N];
int f[N][N][N], g[N][N][N], F[N], G[N];

int main() {
	rd(n, m);
	for (int i = 1; i <= n; i++) rds(a[i], m);
	int tn = n - 1, tm = m - 1;
	while (tn) ++w, tn >>= 1;
	while (tm) ++w, tm >>= 1;
	for (int l = 1; l <= n; l++)
		for (int r = l; r <= n; r++) {
			f[l][r][m+1] = m;
			for (int j = m; j; j--)
				if (!(l == r || (a[l][j] == a[r][j] && f[l][r-1][j] >= j))) f[l][r][j] = j - 1;
				else f[l][r][j] = a[l][j] == a[l][j+1] ? f[l][r][j+1] : j;
		}
	for (int l = 1; l <= m; l++)
		for (int r = l; r <= m; r++) {
			g[l][r][n+1] = n;
			for (int i = n; i; i--)
				if (!(l == r || (a[i][l] == a[i][r] && g[l][r-1][i] >= i))) g[l][r][i] = i - 1;
				else g[l][r][i] = a[i][l] == a[i+1][l] ? g[l][r][i+1] : i;
		}
	if (f[1][n][1] == m) return print(0), 0;
	for (int k = 1; k <= w; k++) {
		for (int l = 1; l <= n; l++)
			for (int r = l; r <= n; r++)
				for (int j = 1; j <= m; j++)
					f[l][r][j] = f[l][r][f[l][r][j]+1];
		for (int l = 1; l <= m; l++)
			for (int r = l; r <= m; r++)
				for (int i = 1; i <= n; i++)
					g[l][r][i] = g[l][r][g[l][r][i]+1];
		for (int i = 1; i <= n; i++)
			for (int j = 1; j <= m; j++) {
				for (int p = i; p <= n; p++) F[p] = f[i][p][j];
				for (int p = j; p <= m; p++) G[p] = g[j][p][i];
				F[n+1] = j - 1, G[m+1] = i - 1;
				for (int p = i; p <= n; p++)
					for (int w = F[p]; w > F[p+1]; w--)
						g[j][w][i] = max(g[j][w][i], p);
				for (int p = j; p <= m; p++)
					for (int w = G[p]; w > G[p+1]; w--)
						f[i][w][j] = max(f[i][w][j], p);
			}
		if (f[1][n][1] == m) return print(k), 0;
	}
	return 0;
}


Measured memory_bytes: 30452